power: add documentation for ACT8945A's charger DT bindings

This patch adds documentation for the DT bindings of the charger
subdevice of ACT8945A MFD.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>