mfd: stpmic1: Use devm_register_power_off_handler()

Simplify the code a bit by using devm_register_power_off_handler(), which is
a purpose-specific wrapper for devm_register_sys_off_handler().  No intended
functional changes are introduced.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Link: https://lore.kernel.org/r/219e0de8bcd1b2ef24142c837d8331ffc535ab26.1731415409.git.dsimic@manjaro.org


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