mfd: max77620: Convert poweroff support to sys-off API

mfd: max77620: Convert poweroff support to sys-off API

Convert max77620_pm_power_off() to the sys-off callback prototype and
register it with the sys-off API when the device tree marks the PMIC as
a system power controller. This also removes the global max77620_scratch
pointer by passing the chip instance through the callback data.

This modernizes the driver's poweroff handling and aligns it with
the kernel sys-off infrastructure.

Signed-off-by: Diogo Ivo <diogo.ivo@tecnico.ulisboa.pt>
Link: https://patch.msgid.link/20260514-smaug-poweroff-v1-2-30f9a4688966@tecnico.ulisboa.pt
Signed-off-by: Lee Jones <lee@kernel.org>