regulator: act8865: add input supply handling

The act88600/act8846/act8865 regulators have a number of input supplies
supplying the individual regulators. This may even be recursively like on
most Rockchip boards using the act8846 where REG4 is most of the time
connected to the inl1-supply.

Therefore add the ability to specify the input supplies for the individual inputs.
The input-names are taken from the datasheets of act8600, act8846 and act8865.
On the act8600 some regulators do not have separate input supplies.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>