xtensa: mmu: add arch_reserved_pages_update

This adds arch_reserved_pages_update() which is called in
k_mem_manage_init() to reserve some physical pages so they
are not re-mapped. This is due to Zephyr's linker scripts
for Xtensa which often puts something before z_mapped_start
(aka .text, for example, vecbase). That space needs to be
reserved or else k_mem_map() would be mapping those that
could result in faults.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>