dev/serial: Move zynq_uart_input_clock()

This allows to wrap this function using the linker.

bsps/include/dev/serial/zynq-uart-regs.h

@@ -176,6 +176,11 @@ void zynq_uart_write_char_polled(volatile zynq_uart *regs, char c);
   */
 void zynq_uart_reset_tx_flush(volatile zynq_uart *regs);
 
+/**
+ * @brief Returns the Zynq UART input clock frequency in Hz.
+ */
+uint32_t zynq_uart_input_clock(void);
+
 /**
  * @brief Calculates the clock and baud divisor of the best approximation of
  *   the desired baud.