drm/bridge/sii8620: add Kconfig dependency on extcon

drm/bridge/sii8620: add Kconfig dependency on extcon

The driver can work with or without extcon framework, but if extcon is
build as module, sii8620 should be build as module as well.

Fixes: 688838442147 ("drm/bridge/sii8620: use micro-USB cable detection logic to detect MHL")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409062708.4326-1-a.hajda@samsung.com
Signed-off-by: Sean Paul <seanpaul@chromium.org>