boards: frdm_mcxn947: Enable CACHE64 FlexSPI cache

1. Enable CACHE64 block used by FlexSPI.
2. LPCAC cache is already enabled by SystemInit which
   is called earlier
3. Configure the FlexSPI clock when running in XIP mode
   as we cannot rely on the driver setting this up as it
   may not be enabled.

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