gpio: pl061: convert to use generic irq chip

Convert the pl061 irq_chip code to use the generic irq chip code.

This has the side effect of using 32-bit accesses rather than 8-bit
accesses to interrupt registers. The h/w TRM and testing seem to indicate
this is fine.

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