gpio: davinci: fix gpio selection for OF

The driver missed an of_xlate function to translate gpio numbers
as found in the DT to the correct chip and number.

While there I've set #gpio_cells to a fixed value of 2.

I've used gpio-pxa.c as template for those changes and tested my changes
successfully on a da850 board using entries for gpio-leds in a DT. So I didn't
reinvent the wheel but just copied and tested stuff.

Thanks to Grygorii Strashko for the hint to the existing code in gpio-pxa.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>