pinctrl: add lpc18xx pinctrl driver

Pinctrl driver for the System Control Unit (SCU) found on NXP
LPC18xx/43xx devices.

Driver uses the generic pinctrl DT bindings for multiplexing
and property settings.

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