mfd: rohm-bd71828: Add power off functionality

Since the chip can power off the system, add the corresponding
functionality.
Based on https://github.com/kobolabs/Kobo-Reader/raw/master/hw/imx6sll-clara2e/kernel.tar.bz2



Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/20240402111700.494004-3-andreas@kemnade.info


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