gpio: gpio-lpc32xx: Add gpio_to_irq mapping

This patch helps mapping with gpio_to_irq for the GPIOs that are irq enabled.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Tested-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>