mfd: axp20x: Switch to the sys-off handler API

This removes a layer of indirection through pm_power_off() and allows
the PMIC handler to be used as a fallback when firmware power off fails.
This happens on boards like the Clockwork DevTerm R-01 where OpenSBI
does not know how to use the PMIC to power off the board.

Move the check for AXP288 to avoid registering a dummy handler.

Signed-off-by: Samuel Holland <samuel@sholland.org>
[Lee: Removed superfluous new line]
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221228162752.14204-1-samuel@sholland.org