EmbeddedPkg/DmaLib: add routine to allocate aligned buffers

EmbeddedPkg/DmaLib: add routine to allocate aligned buffers

DmaLib's purpose is to manage memory that is shared between the host
and DMA capable devices. In some cases, this requires a larger alignment
than page size, and we currently don't cater for that in DmaLib. So add
a variant of DmaAllocateBuffer () that takes an alignment parameter.

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