gpio: hlwd: Add basic IRQ support

This patch implements level-triggered IRQs in the Hollywood GPIO driver.
Edge triggered interrupts are not supported in this GPIO controller, so
I moved their emulation into a separate patch.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>