Platform: AN521: Set proper MAIR_ATTR for MPU regions

MAIR_ATTR for all regions are set as ZERO, which indicates
all regions as Device-nGnRnE. Set proper MAIR_ATTR settings
for dedicated MPU regions.
The attributes come from the default memory map. And set the
same attribute for Outer and Inner of Normal memory.

Device-nGnRE : 0b [Outer]0000 [Inner]0100
Code WT   RA : 0b [Outer]1010 [Inner]1010
DATA WBWA RA : 0b [Outer]1111 [Inner]1111

Change-Id: I2c669b693890e722219f2951fb5f72b61a411cc6
Signed-off-by: Ken Liu <ken.liu@arm.com>