pinctrl: lpc18xx: add support for usb1 pinconf

The dedicated USB1 pins can be configured with pull-down and
for low power mode (suspend). Add support for this in the
pinctrl driver.

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