regulator: mp8859: add supply entry

Add vin_supply to the regulator description to support a nice
regulator tree.

Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20200203110034.1448-1-m.reichl@fivetechno.de


Signed-off-by: Mark Brown <broonie@kernel.org>