power/reset: Add a new driver implementing 'power off by restarting'

Some devices, Buffalo Linkstation LS-XHL and LS-CHLv2 for example,
power-off by restarting to letting u-boot hold the SoC until the user
presses a key. Add a generic driver to implement this. It binds a function
to pm_power_off, which calls arm_pm_restart.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>