Define the MPUIR register for MPU supporting ARMv6 and onwards.
Currently we only support unified MPU.

The size of the unified MPU is defined via the number of "dregions".
So just a single config is added to specify this size. (When split MPU
is implemented we will add an extra iregions config).
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 9f248950b803a08c8b3c978931663182f7e882e7.1434501320.git.peter.crosthwaite@xilinx.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>