power/reset: Add a new driver to turn QNAP board power off

The QNAP NAS boxes have a microcontroller attached to the SoCs second
serial port. By sending it a simple command, it will turn the power for
the board off. This driver registers a function for pm_power_off to send
such a command.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>