mfd: tps65219: Add support for soft shutdown via sys-off API

Use new API for power-off mode support:
Link: https://lwn.net/Articles/894511/
Link: https://lore.kernel.org/all/7hfseqa7l0.fsf@baylibre.com/



sys-off API allows support of shutdown handler and restart handler.

Shutdown was not supported before that enhancement.
This is required for platform that are not using PSCI.

Test:
- restart:
  # reboot
  Default is cold reset:
  # cat /sys/kernel/reboot/mode
  Switch boot mode to warm reset:
  # echo warm > /sys/kernel/reboot/mode
- power-off:
  # halt

Tested on AM62-LP-SK board.

Signed-off-by: Jerome Neanne <jneanne@baylibre.com>
Suggested-by: Andrew Davis <afd@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230608071947.3467751-1-jneanne@baylibre.com