devicetree: bindings: pinctrl: Add binding for ti,da850-pupd

Device-tree bindings for TI DA850/OMAP-L138/AM18XX pullup/pulldown
pinconf controller.

Signed-off-by: David Lechner <david@lechnology.com>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>