pinctrl: lpc18xx: add nxp,gpio-pin-interrupt property

Add support for setting up GPIO pin interrupts in the lpc18xx pinctrl
driver. The LPC18xx SCU contain two registers that sets up the signal
routing to the GPIO pin interrupt (PINT) block. The routing uses the
GPIO namespace and not the pin namespace so a lookup is preformed on
the pin.

Routing configuration is done in the device tree by using the new
nxp,gpio-pin-interrupt property. This property takes single parameter
which sets the PINT hwirq for the GPIO.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>