arm: Add option to skip buffer zeroing

The DMA framework currently zeros all buffers because it (righfully so)
assumes that drivers will soon need to pass the memory to a device.
Some devices/use case may not require zeroed memory and there can
be an increase in performance if we skip the zeroing. Add a DMA_ATTR
to allow skipping of DMA zeroing.

Change-Id: Id9ccab355554b3163d8e7eae1caa82460e171e34
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>