dt-bindings: drm/bridge/synopsys: dw-hdmi: Add "unwedge" for ddc bus

In certain situations it was seen that we could wedge up the DDC bus
on the HDMI adapter on rk3288.  The only way to unwedge was to mux one
of the pins over to GPIO output-driven-low temporarily and then
quickly mux back.  Full details can be found in the patch
("drm/bridge/synopsys: dw-hdmi: Add "unwedge" for ddc bus").

Since unwedge requires remuxing the pins, we first need to add to the
bindings so that we can specify what state the pins should be in for
unwedging.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190502225336.206885-1-dianders@chromium.org