dmaengine: xilinx: Rename driver and config

In the existing vdma driver support for
AXI DMA and CDMA got added so the driver is no
longer VDMA specific.

This patch renames the driver and DT binding doc to xilinx_dma
and updates the Kconfig description for all the DMAS.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>