dt-bindings: add the grf clock for dw-mipi-dsi

For RK3399, the grf clock should be controlled by dw-mipi-dsi driver,
add the description for this clock.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1490147691-4489-3-git-send-email-zyw@rock-chips.com