[ARM] Always mark ARMv6 PTWs outer cacheable

Other platforms other than SMP may have an outer cache.  For these, we
also need to mark the page table walks outer cacheable.  Since marking
the walks always outer cacheable apparantly has no side effects, we
might as well always mark them so.

However, we continue to only mark PTWs shared if we have SMP enabled.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>