arch: arm: mpu: Make XN bit conditional on CONFIG_XIP for RAM region

Make the execute never (XN) bit conditonal on CONFIG_XIP for the RAM MPU
region attribute. This is required because when CONFIG_XIP is not set,
the entire image will be linked into SRAM. In this case, SRAM must be
executable.

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