gpio: pl061: fix IRQ handling for GPIOs >= PL061_GPIO_NR

IRQ handling is wrong for any GPIO >= PL061_GPIO_NR.

Fix this by implementing and using a proper .to_irq method.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>