ArmPkg/AArch64Mmu: Fix XN attribute for device memory

ArmPkg/AArch64Mmu: Fix XN attribute for device memory

Now XN attribute will be set automatically if the region is declared
as device memory. However, the function ArmMemoryAttributeToPageAttribute
is to get attribute for block and page descriptors, not for table
descriptors, so attribute TT_TABLE_*XN does not really take effect.

Need to use TT_*XN_MASK instead.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>