mfd: dt-bindings: Add RK805 device tree bindings document

Add device tree bindings documentation for Rockchip's RK805 PMIC.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Joseph Chen <chenjh@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>