pinctrl: samsung: Add GPIO to IRQ translation

Some drivers require a way to translate GPIO pins to their IRQ numbers.

This patch adds the .to_irq() gpiolib callback to pinctrl-samsung
driver, which creates (if not present yet) and returns an IRQ mapping
for given GPIO pin.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>