mfd: bd71828, bd71815: Prepare for power-supply support

Add core support for ROHM BD718(15/28/78) PMIC's charger blocks.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Link: https://lore.kernel.org/r/20250821-bd71828-charger-v3-1-cc74ac4e0fb9@kemnade.info


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