ArmPkg/Mmu: set required XN attributes for device mappings

ArmPkg/Mmu: set required XN attributes for device mappings

To prevent speculative intruction fetches from MMIO ranges that may
have side effects on reads, the architecture requires device mappings
to be created with the XN or UXN/PXN bits set (for the ARM/EL2 and
EL1&0 translation regimes, respectively.)

Note that, in the ARM case, this involves moving all accesses to a
client domain since permission attributes like XN are ignored from
a manager domain. The use of a client domain is actually mandated
explicitly by the UEFI spec.

Reported-by: Heyi Guo <heyi.guo@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18891 6f19259b-4bc3-4df7-8a09-765794883524