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.

Note: only the header file was modified to add the enum to allow the code to
compile, however the arm32 implementation was not pulled, and the arm64
implementation is missing as well

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>