power: supply: max17042: add platform driver variant

power: supply: max17042: add platform driver variant

Maxim PMICs may include fuel gauge with additional features, which is
out of single Linux power supply driver scope.

For example, in max77705 PMIC fuelgauge has additional registers, like
IIN_REG, VSYS_REG, ISYS_REG. Those needed to measure PMIC input current,
system voltage and current respectively. Those measurements cannot be
bound to any of fuelgauge properties.

The solution here add and option to use max17042 driver as a MFD sub
device, thus allowing any additional functionality be implemented as
another sub device. This will help to reduce code duplication in MFD
fuel gauge drivers.

Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Link: https://lore.kernel.org/r/20250107-b4-max17042-v6-2-3d0104ad5bc7@gmail.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>