Add support for the Analog Devices AXI-DMAC DMA controller. This controller
is a soft peripheral that can be instantiated in a FPGA and is often used
in Analog Devices' reference designs for FPGA platforms.

The peripheral has various configuration options that can be selected at
synthesis time and influence the supported features of the instantiated
peripheral, those options are represented as device-tree properties to
allow the driver to behave accordingly.

The peripheral has a zero latency architecture, which means it is possible
to switch from one to the next descriptor without any delay. This is
archived by having a internal queue which can hold multiple descriptors.
The driver supports this, which means it will submit new descriptors
directly to the hardware until the queue is full and not wait for a
descriptor to complete before the next one is submitted. Interrupts are
used for the descriptor queue flow control.

Currently the driver supports SG, cyclic and interleaved slave DMA.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>