lib: extmem: Add ext_mem_get_obj_size and requirement on ext_mem_obj_initialize

This commit adds a new function for getting the size of an
external memory object. This function takes a vmm_obj
argument which must be part of an ext_mem_obj.
ext_mem_obj_get_size validates this by checking the
vmm_obj_ops get_page pointer which must now point to
ext_mem_obj_get_page when initializing an ext_mem_obj.

Bug: 379677575
Change-Id: I5dfc7e8eff4de4e2f0bccf3f8f7220fbadbcd834
2 files changed