gpio: tps65912: Add GPIO driver for the TPS65912 PMIC

This patch adds support for the TPS65912 PMIC GPIOs.

TPS65912 has five configurable GPIOs that can be used for several
purposes.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>