gpio: davinci: add support for pinmux gpio ranges

This adds support for the pinmux gpio ranges feature to the DaVinci gpio
driver. Only device tree is supported since the non-DT boards don't
use a generic pinmux controller.

Cc: Keerthy <j-keerthy@ti.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>