power: reset: add driver for LinkStation power off

Some Buffalo LinkStations perform the power off operation, at restart
time, depending on the state of an output pin (LED2/INTn) at the ethernet
PHY. This pin is also used to wake the machine when a WoL packet is
received by the PHY.

The driver is required by the Buffalo LinkStation LS421DE (ARM MVEBU),
and other models. Without it, the board remains forever halted if a
power off command is executed, unless the PSU is disconnected and
connected again.

Add the driver to provide the power off function and also make the WoL
feature to be available.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>