gpio: AMD G-Series PCH gpio driver

GPIO platform driver for the AMD G-series PCH (eg. on GX-412TC)

This driver doesn't registers itself automatically, as it needs to
be provided with platform specific configuration, provided by some
board driver setup code.

Didn't implement oftree probing yet, as it's rarely found on x86.

Cc: linux-gpio@vger.kernel.org
Cc: linus.walleij@linaro.org
Cc: bgolaszewski@baylibre.com
Cc: dvhart@infradead.org
Cc: platform-driver-x86@vger.kernel.org
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>