ARM: 8734/1: mm: idmap: Mark variables as ro_after_init

idmap_pgd, arch_phys_to_idmap_offset are setup once
while init stage, and never changed after that.
so, it is good candidate for __ro_after_init.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jinbum Park <jinb.park7@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>