[PATCH] pio: add arch specific gpio_is_valid() function

[David Brownell]

Still not liking or accepting this proposed
change to the GPIO framework.

For the AT91 case (where integers 0..N are
IRQs, but N..max are GPIOs)

A simpler solution is just to use a bit in
the integer to indicate IRQ vs GPIO.  Like
maybe the sign bit.. which is never set on
valid GPIO numbers, but platforms could let
be set on IRQs.