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: 68883844 ("drm/bridge/sii8620: use micro-USB cable detection logic to detect MHL")
Reported-by: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NAndrzej Hajda <a.hajda@samsung.com>
Reviewed-by: NChanwoo Choi <cw00.choi@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409062708.4326-1-a.hajda@samsung.comSigned-off-by: NSean Paul <seanpaul@chromium.org>

drivers/gpu/drm/bridge/Kconfig

@@ -74,6 +74,7 @@ config DRM_SIL_SII8620
 	tristate "Silicon Image SII8620 HDMI/MHL bridge"
 	depends on OF && RC_CORE
 	select DRM_KMS_HELPER
+	imply EXTCON
 	help
 	  Silicon Image SII8620 HDMI/MHL bridge chip driver.