mfd: tps6586x: Use devm-based power off handler

Convert the power off handler to a devm-based power off handler.

Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Benjamin Bara <benjamin.bara@skidata.com>
Link: https://lore.kernel.org/r/20230327-tegra-pmic-reboot-v7-4-18699d5dcd76@skidata.com


Signed-off-by: Lee Jones <lee@kernel.org>