mfd: Add documentation for ACT8945A DT bindings

The Active-semi ACT8945A PMIC is a Multi-Function Device, it has
two subdevices:
 - Regulator
 - Charger

This patch adds documentation for ACT8945A DT bindings.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>