gpio: pl061: use chained_irq_* functions in irq handler

Use chained_irq_enter/exit helper functions instead of direct pointer
accesses. This is needed for generic irq chip conversion.

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>