pinctrl: lpc1850-scu: document nxp,gpio-pin-interrupt

Update devicetree documention for lpc1850-scu with the new
nxp,gpio-pin-interrupt property.

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