ArmPkg/ArmDmaLib: deal with NULL return value of UncachedAllocatePages ()

ArmPkg/ArmDmaLib: deal with NULL return value of UncachedAllocatePages ()

The allocation function UncachedAllocatePages () may return NULL, in
which case our implementation of DmaAllocateBuffer () should return
EFI_OUT_OF_RESOURCES rather than silently ignoring the NULL value and
returning EFI_SUCCESS.

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