drm/bridge/sii8620: add reading device capability registers

This functionality is necessary to implement MHL3 modes.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-4-git-send-email-a.hajda@samsung.com