mfd: dt-bindings: Add RK808 device tree bindings document

Add device tree bindings documentation and a header file
for rockchip's RK808 pmic.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Zhang Qing <zhangqing@rock-chips.com>
Tested-by: Heiko <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>