gpio: Add a driver for Cortina Systems Gemini GPIO

This is a heavy edit/rewrite of the GPIO driver for the Gemini
SoC from arch/arm/mach-gemini/gpio.c.

This rewrite uses all the best-in-class helper like generic
GPIO and GPIOLIB_IRQCHIP and has been tested on ITian Square One
Gemini-based NAS/router.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>