dmaengine: xilinx: Add empty device_config function

Various DMA users call the dmaengine_slave_config() and expect it to
succeed, but that can only succeed if .device_config is implemented.
Add empty device_config function rather than patching all the places
which use dmaengine_slave_config().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Kedareswara rao Appana <appana.durga.rao@xilinx.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20210804195140.61396-1-marex@denx.de


Signed-off-by: Vinod Koul <vkoul@kernel.org>