ARM: 8642/1: LPAE: catch pending imprecise abort on unmask

Asynchronous external abort is coded differently in DFSR with LPAE enabled.

Fixes: 9254970c

 "ARM: 8447/1: catch pending imprecise abort on unmask".
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>