ArmVirtPkg/ArmVirtPlatformLib: map executable NOR region as normal memory

ArmVirtPkg/ArmVirtPlatformLib: map executable NOR region as normal memory

The ARM architecture version 7 and later mandates that device mappings
have the XN (non-executable) bit set, to prevent speculative instruction
fetches from read-sensitive regions. This implies that we should not map
regions as device if we want to execute from them, so the NOR region that
contains our FD image should be mapped as normal memory instead.

The MMU code deals correctly with overlapping ARM_MEMORY_REGION_DESCRIPTOR
entries, and later entries in the array take precedence over earlier ones.
So simply add an entry to the end of the array that overrides the mapping
attributes of the FD image, wherever it resides.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18889 6f19259b-4bc3-4df7-8a09-765794883524