dmaengine: at_xdmac: add support for sama7g5 based at_xdmac

SAMA7G5 SoC uses a slightly different variant of the AT_XDMAC.
Added support by a new compatible and a layout struct that copes
to the specific version considering the compatible string.
Only the differences in register map are present in the layout struct.
I reworked the register access for this part that has the differences.
Also the Source/Destination Interface bits are no longer valid for this
variant of the XDMAC. Thus, the layout also has a bool for specifying
whether these bits are required or not.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20201016093850.290053-1-eugen.hristev@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>