power: reset: Add poweroff driver for ATC260x PMICs

This driver provides poweroff and reboot support for a system through
the ATC2603C and ATC2609A chip variants of the Actions Semi ATC260x
family of PMICs.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>