regulator: act8865: add act8846 to DT binding documentation

This patch adds a new "active-semi,act8846" compatible string and a
list of supported regulator names to the devicetree binding
documentation for Active-Semi PMUs.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>