doc: dt: add documentation for lpc1850-rgu reset driver

Add device tree binding documentation for the Reset Generation Unit
(RGU) found on NXP LPC18xx and LPC43xx devies.

This documentation also includes a table which shows the RGU reset
number and the connected peripheral.

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