GPIO: sa1100: implement proper gpiolib gpio_to_irq conversion

The existing gpio_to_irq() implementation on sa1100 only translates
validly for internal GPIOs.  Since this sub-arch enables GPIOLIB
support, this results in buggy translations for non-internal GPIOs.

Get rid of the private gpio_to_irq() implementation, replacing it
with the .to_irq method in the sa1100 gpio chip instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>