gpio: grgpio: Add irq support

The drivers sets up an irq domain and hands out unique irqs to irq
capable gpio lines regardless of how underlying irq maps to gpio
lines. Any gpio line can map to any one or none of the irqs of the
core, independently of each other.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>