ArmPkg/ArmMmuLib: Add new attribute WRITE_BACK_NONSHAREABLE

ArmPkg/ArmMmuLib: Add new attribute WRITE_BACK_NONSHAREABLE

Flash region needs to be set as cacheable (write back) to increase
performance, if PEI is still XIP on flash or DXE FV is decompressed
from flash FV. However some ARM platforms do not support to set flash
as inner shareable since flash is not normal DDR memory and it will
not respond to cache snoop request, which will causes system hang
after MMU is enabled.

So we need a new ARM memory region attribute WRITE_BACK_NONSHAREABLE
for flash region on these platforms specifically. This attribute will
set the region as write back but not inner shared.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Peicong Li <lipeicong@huawei.com>
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>