bq25890_charger: enter ship mode on power off

This sets the charger device into 'ship mode' when powering off by
hooking onto pm_power_off. 'Ship mode' is skipped if the device is
actively charging. This feature is disabled by default, and can be
enabled by setting 'ship_mode=1' when loading this module.

Since the poweroff can be aborted (i.e. when charging), any previous
value for pm_power_off is saved and run in the cases where the
bq25890 is *not* used to power off system.
2 jobs for byzantium_bq25890_ship_mode in 64 minutes and 8 seconds (queued for 2 seconds)