[bsp][nxp] add missing drv_spi.h

Otherwise when building spi users such as u8g2, gcc will complain can't
find the drv_spi.h.

bsp/nxp/mcx/mcxn/Libraries/drivers/drv_spi.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2006-2024, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Change Logs:
+ * Date           Author          Notes
+ * 2024-03-22     Jisheng Zhang   The first version for mcxn
+ */
+
+#ifndef __DRV_SPI_H__
+#define __DRV_SPI_H__
+
+#include <rtdevice.h>
+
+rt_err_t rt_hw_spi_device_attach(const char *bus_name, const char *device_name, rt_base_t cs_pin);
+
+#endif /*__DRV_SPI_H__ */