dt-bindings: mfd: rk808: Add binding information for RK809 and RK817.

Add device tree bindings documentation for Rockchip's RK809 & RK817 PMIC.

Signed-off-by: Tony Xie <tony.xie@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>