samples: drivers: memc: add support for RW612 BGA board

Since the RW612 BGA board's FlexSPI configuration prevents the board
from executing from flash while the PSRAM is configured (unless both are
configured by a bootloader prior to XIP), run the MEMC sample from
internal code SRAM, and write data to the external PSRAM block via the
FlexSPI.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>