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: Ib78a19cb341c3c441f91d5b004c6375c80c10413
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
arm: Skip DMA zeroing completely only if specified

With the dma attribute 'SKIP_ZEROING', dma allocation skips memset and
also cache flush operations. This leaves dirty lines in the caches which
when evicted later would cause issue to the allocated buffer. For some
clients this may be okay but other clients need to do the actual sync.
Switch the code to only skip __dma_clear_buffer if both
DMA_ATTR_SKIP_ZEROING and DMA_ATTR_SKIP_CPU_SYNC are specified. If only
DMA_ATTR_SKIP_ZEROING is specified, just skip the zeroing but still do the
cache operations.

Change-Id: I81dd6bbeed7ec7f1dce28cd8d0aba7c8900ec550
Signed-off-by: Taniya Das <tdas@codeaurora.org>
[lauraa@codeaurora.org: Context differences]
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>