gpio: Add Cadence GPIO driver

This patch adds a driver for Cadence GPIO controller.

It can be enabled with GPIO_CADENCE Kconfig option.
It uses generic GPIO infrastructure and works
as an interrupt controller.
At the moment it only supports level sensitive irqs.

Signed-off-by: Jan Kotas <jank@cadence.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>