ArmPkg/CpuDxe ARM: honour RO/XP attributes in SetMemoryAttributes()

Enable the use of strict memory permissions on ARM by processing the
EFI_MEMORY_RO and EFI_MEMORY_XP rather than ignoring them. As before,
calls to CpuArchProtocol::SetMemoryAttributes that only set RO/XP
bits will preserve the cacheability attributes. Permissions attributes
are not preserved when setting the memory type only: the way the memory
permission attributes are defined does not allows for that, and so this
situation does not deviate from other architectures.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>