dt-bindings: add power domain node for dw-mipi-rockchip

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1487577744-2855-6-git-send-email-zyw@rock-chips.com