mmc: sh_mmcif: Document DT bindings

The sh-mmcif driver implements DT support but the bindings are not
documented. Document them.

Cc: devicetree@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>