ARM: mm: set DOMAIN_WR_RARE for rodata

This creates DOMAIN_WR_RARE for the kernel's .rodata section, separate
from DOMAIN_KERNEL to avoid predictive fetching in device memory during
a DOMAIN_MANAGER transition.

TODO: handle kernel module vmalloc memory, which needs to be marked as
DOMAIN_WR_RARE too, for module .rodata sections.

Signed-off-by: Kees Cook <keescook@chromium.org>