ARM: 7041/1: gpio-ep93xx: hookup the to_irq callback in the driver

Remove the ep93xx machine specific dependencies for gpio_to_irq() by
hooking up the callback in the driver and using __gpio_to_irq.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>