dmaengine: at_xdmac: add AXI priority support and recommended settings

The sama7g5 version of the XDMAC supports priority configuration and
outstanding capabilities.
Add defines for the specific registers for this configuration, together
with recommended settings.
However the settings are very different if the XDMAC is a mem2mem or a
per2mem controller.
Thus, we need to differentiate according to device tree property.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20201016093918.290137-1-eugen.hristev@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>