power: reset: reboot-mode: Add managed resource API

Provide managed resource version of reboot_mode_register() and
reboot_mode_unregister() to simplify implementations.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>