power: reset: linkstation-poweroff: prepare for new devices

This commit prepare driver for another device support.

New power_off_cfg structure describes two most important things: name of
mdio bus and pointer to register setting function. It allow to add new
device with different mdio bus node and other phy register config.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>