dt-bindings: sdhci-omap: Add pinctrl bindings

Add pinctrl binding rquired to get the mux mode and IODelay
values from devicetree.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>