EmbeddedPkg: rename NullDmaLib to CoherentDmaLib

EmbeddedPkg: rename NullDmaLib to CoherentDmaLib

The name NullDmaLib suggests that this library is a placeholder that
only exists to fulfil formal dependencies on the DmaLib library class
without providing an actual implementation. This is not the case, though:
NullDmaLib does implement DmaLib fully, but doing so simply requires very
little effort on a cache coherent platform. So let's rename it to
CoherentDmaLib instead.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>