Documentation: devicetree: bindings: dma: Add properties for DMA alignment

Documentation: devicetree: bindings: dma: Add properties for DMA alignment

Adds the pixels per clock and dma-align device tree properties. These
are used to determine the DMA memory alignment.
New compatible strings xlnx,axi-frmbuf-rd-v2.1 and
xlnx,axi-frmbuf-wr-v2.1 have also been added for this.

Signed-off-by: Vishal Sagar <vishal.sagar@xilinx.com>
Reviewed-by: Radhey Shyam Pandey <radheys@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>