Add documentation for 'xlnx,zynqmp-8.9a' SDHCI controller and optional
properties followed by example.
Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>