ARM: 8732/1: NOMMU: Allow userspace to access background MPU region

Currently, with MPU enabled, we prohibit userspace access to anything
except RAM. Benjamin, reported that because of that his userspace
application cannot access framebuffer's memory he reserved in device
tree. It turns out we have no option other than to allow userspace
access memory covered by background region.

Reported-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Tested-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>