dt-bindings: mmc: arasan: Update Documentation for the input clock

Add documentation for an optional input clock which is essentially used
in sampling the input data coming from the card.

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>