power: reset: syscon-reboot-mode: Use managed resource API

Use the managed resource version of reboot_mode_register().

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>