mfd: dt-bindings: Add RK818 device tree bindings document

Add device tree bindings documentation for Rockchip's RK818 PMIC.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>