gpio: pl061: convert to use 0 for no irq

We don't want drivers using NO_IRQ, so remove its use. For now, 0 or
-1 means no irq until platforms are converted to use 0.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Linus Walleij <linus.ml.walleij@gmail.com>