dmaengine: axi-dmac: add regmap support

The registers for AXI DMAC are detailed at:
  https://wiki.analog.com/resources/fpga/docs/axi_dmac#register_map

This change adds regmap support for these registers, in case some wants to
have a more direct access to them via this interface.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
[vkoul: fixed code style issue]
Signed-off-by: Vinod Koul <vkoul@kernel.org>