tile: handle __ro_after_init like parisc does

The tile architecture already marks RO_DATA as read-only in
the kernel, so grouping RO_AFTER_INIT_DATA with RO_DATA, as is
done by default, means the kernel faults in init when it tries
to write to RO_AFTER_INIT_DATA.  For now, just arrange that
__ro_after_init is handled like __write_once, i.e. __read_mostly.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>