ArmPkg/ArmDmaLib: interpret GCD attributes as a bit field

ArmPkg/ArmDmaLib: interpret GCD attributes as a bit field

Comparing a GCD attribute field directly against EFI_MEMORY_UC and
EFI_MEMORY_WT is incorrect, since it may have other bits set as well
which are not related to the cacheability of the region. So instead,
test explicitly against the flags EFI_MEMORY_WB and EFI_MEMORY_WT,
which must be set if the region may be mapped with cacheable attributes.

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