dm: implement a DMA uclass

Implement a DMA uclass so that the devices like ethernet, spi,
mmc etc can offload the data transfers from/to the device and
memory.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jteki@openedev.com>