pinctrl: bindings: Add OMAP pinctrl binding

Add basic skeleton of OMAP pinctrl bindings. This is compatible with
pinctrl,single bindings and is meant purely as a reference point.
Acked-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NNishanth Menon <nm@ti.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/ti,omap-pinctrl.txt

+OMAP Pinctrl definitions
+
+Required properties:
+- compatible : Should be one of:
+  "ti,omap2420-padconf" - OMAP2420 compatible pinctrl
+  "ti,omap2430-padconf" - OMAP2430 compatible pinctrl
+  "ti,omap3-padconf" - OMAP3 compatible pinctrl
+  "ti,omap4-padconf" - OMAP4 compatible pinctrl
+  "ti,omap5-padconf" - OMAP5 compatible pinctrl
+
+See Documentation/devicetree/bindings/pinctrl/pinctrl-single.txt for further details.