soc: arm: nxp_imx: rt: enable SOC fixed MPU regions, add SDRAM0 region

Enable SOC fixed MPU regions by default for the RT10xx/RT11xx SOC lines.

Additionally, add code to handle defining the SDRAM0 region as
device type (non cacheable, non shareable). This behavior can
be disabled with CONFIG_NXP_IMX_EXTERNAL_SDRAM=y. Set this Kconfig
for all boards in tree using SDRAM.

This will resolve an issue present on the RT11xx series where
the core may execute speculative prefetches to the SDRAM region when
no SDRAM is present on the board, resulting in the system faulting.

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