power: ab8500: Require device tree

The core AB8500 driver and the whole platform is completely
dependent on being probed from device tree so remove the
non-DT probe paths.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>