mmu: add CONFIG_SRAM_REGION_PERMISSIONS

If CONFIG_MMU is active, choose whether to separate text,
rodata, and ram into their own page-aligned regions so that
they have have different MMU permissions applied.

If disabled, all RAM pages will have RWX permission to
supervisor mode, but some memory may be saved due to lack
of page alignment padding between these regions.

This used to always happen. This patch adds the Kconfig,
linker script changes to come in a subsequent patch.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>