drivers/timers/ptp: support ptp clock dummy driver

This patch adds a dummy PTP clock driver implementation that provides
a software-based PTP clock for testing and development purposes.

Key changes include:

1. Dummy PTP clock driver implementation:
   - Added drivers/timers/ptp_clock_dummy.c
   - Provides software-based PTP clock using system monotonic clock
   - Implements all required lower-half driver operations

2. Lower-half driver operations:
   - adjfine: Frequency adjustment (stores adjustment value)
   - adjtime: Time offset adjustment (applies delta to base time)
   - gettime: Returns current PTP clock time
   - settime: Sets PTP clock time
   - getcaps: Reports clock capabilities (1 billion PPB max adjustment)
   - getcrosststamp: Provides system/device cross-timestamp

3. Driver initialization:
   - Added ptp_clock_dummy_init() in drivers/drivers_initialize.c
   - Auto-registration under CONFIG_PTP_CLOCK_DUMMY configuration
   - Creates /dev/ptp0 character device node

4. Build system integration:
   - Added CONFIG_PTP_CLOCK_DUMMY Kconfig option (depends on PTP_CLOCK)
   - Updated CMakeLists.txt and Make.defs
   - Added include/nuttx/timers/ptp_clock_dummy.h header

5. Implementation details:
   - Uses clock_gettime(CLOCK_MONOTONIC) as time base
   - Tracks time offset and frequency adjustment internally
   - Suitable for testing PTP clock applications without hardware

This dummy driver enables development and testing of PTP clock applications
on platforms without dedicated PTP hardware support.

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

include/nuttx/timers/ptp_clock_dummy.h

@@ -0,0 +1,74 @@
+/****************************************************************************
+ * include/nuttx/timers/ptp_clock_dummy.h
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+#ifndef __INCLUDE_NUTTX_TIMERS_PTP_CLOCK_DUMMY_H
+#define __INCLUDE_NUTTX_TIMERS_PTP_CLOCK_DUMMY_H
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Inline Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Public Types
+ ****************************************************************************/
+
+/****************************************************************************
+ * Public Function Prototypes
+ ****************************************************************************/
+
+#undef EXTERN
+#if defined(__cplusplus)
+#define EXTERN extern "C"
+extern "C"
+{
+#else
+#define EXTERN extern
+#endif
+
+/****************************************************************************
+ * Name: ptp_clock_dummy_initialize
+ *
+ * Description:
+ *   This function will initialize dummy ptp device driver for test.
+ *
+ * Input Parameters:
+ *   devno - The user specifies number of device. ex: /dev/ptpX.
+ *
+ * Returned Value:
+ *   OK if the driver was successfully initialize; A negated errno value is
+ *   returned on any failure.
+ *
+ ****************************************************************************/
+
+int ptp_clock_dummy_initialize(int devno);
+
+#undef EXTERN
+#if defined(__cplusplus)
+}
+#endif
+#endif /* __INCLUDE_NUTTX_TIMERS_PTP_CLOCK_DUMMY_H */