dt-bindings: add rk3399 support for dw-mipi-rockchip

The dw-mipi-dsi of rk3399 is almost the same as rk3288, the rk3399 has
additional phy config clock.

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/1487577744-2855-2-git-send-email-zyw@rock-chips.com