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: Tony Lindgren <tony@atomide.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>