boards: frdm_mcxn947: Add clock setup for FlexSPI

The MEMC driver in memc_mcux_flexspi.c is initialized
before the FlexSPI driver (flash_mcux_flexspi_nor.c)
and hangs during FlexSPI init. Initialize the FlexSPI
clock to 50MHz before the speed is set to the optimum
speed by the FlexSPI driver.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>

boards/nxp/frdm_mcxn947/board.c

@@ -4,6 +4,7 @@
  */
 #include <zephyr/init.h>
 #include <zephyr/device.h>
+#include <zephyr/dt-bindings/clock/mcux_lpc_syscon_clock.h>
 #include <fsl_clock.h>
 #include <fsl_spc.h>
 #include <soc.h>
@@ -191,6 +192,14 @@ static int frdm_mcxn947_init(void)
 	CLOCK_AttachClk(kPLL0_to_CTIMER4);
 #endif
 
+#if CONFIG_FLASH_MCUX_FLEXSPI_NOR
+	/* We downclock the FlexSPI to 50MHz, it will be set to the
+	 * optimum speed supported by the Flash device during FLEXSPI
+	 * Init
+	 */
+	flexspi_clock_set_freq(MCUX_FLEXSPI_CLK, MHZ(50));
+#endif
+
 	/* Set SystemCoreClock variable. */
 	SystemCoreClock = CLOCK_INIT_CORE_CLOCK;