drm/bridge/sii8620: initial support for eCBUS-S mode

The Single-ended eCBUS Mode (eCBUS-S) mode provides 60 Mb/s full-duplex
bidirectional traffic for three channels:
 - CBUS data (CBUS1 channel),
 - High-bandwidth MHL data (eMSC channel),
 - tunneling data (T-CBUS channel).
It is required to fully support MHL3 dongles.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-6-git-send-email-a.hajda@samsung.com