dt-bindings: mmc: arasan: Document 'xlnx,versal-8.9a' controller

Add documentation for 'xlnx,versal-8.9a' SDHCI controller followed by
example.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1586195015-128992-2-git-send-email-manish.narani@xilinx.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>