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>