zephyr: boards: Add files needed for NXP FRDM MCXN947 QSPI variant

1. set BOOT_MAX_IMG_SECTORS value for frmd_mcxn947_qspi. W25Q64 flash on the board
   is very large (8MB), so we must increase the number of max sectors when
   targeting this board with MCUboot.
2. Set the zephyr,flash chosen node to point to internal flash as this board
   supports booting from internal flash only.

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