initialize_mmu sets up atomctl SR which is needed for s32c1i to function
correctly even in noMMU configurations. It's also a good place to set up
caching attributes of physical memory.

Allow enabling INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX in noMMU
configurations for setting up atomctl and cache attributes.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>