[ARM] 3378/1: ixp2000: fix gpio interrupt handling

Patch from Lennert Buytenhek

ixp2000 used to initially mark GPIO interrupts as invalid, and not
mark them valid until set_irq_type() was called, but this doesn't
work if you want to use request_irq() with the SA_TRIGGER_* flags.

So, just mark the GPIO interrupts valid from the beginning.  We
configure GPIOs as inputs when set_irq_type() is called anyway, so
this shouldn't be a problem.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>