reset: add driver for lpc18xx rgu

Add reset driver for the Reset Generation Unit (RGU) found on NXP
LPC18xx and LPC43xx devies. This reset controller features up to 64
reset lines connected to different blocks and peripheral in the SoC.
Most reset lines on the controller are self clearing except for
those dealing with the Cortex-M0 cores on LPC43xx devices.

This driver also registers a restart handler that can be used to
reset the entire device.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>