regulator: dt-bindings: Document charger-supply for max8997

Add charger-supply optional property to enable charging control.

Signed-off-by: Timon Baetz <timon.baetz@protonmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20210130172747.2022977-2-timon.baetz@protonmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>