dt-bindings: add Silicon Image SiI8620 bridge bindings

SiI8620 transmitter converts eTMDS/HDMI signal to MHL 3.0. It is controlled
via I2C bus.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1475823762-16237-3-git-send-email-a.hajda@samsung.com