power: Add simple poweroff-gpio driver

Given appropriate devicetree bindings, this driver registers a
pm_power_off function to set a GPIO line high/low to power down
your board.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Simon Baatz <gmbnomis@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>