sh_mobile_lcdc: Add display notify callback to sh_mobile_lcdc_chan

The callback implements 3 notification events:

- SH_MOBILE_LCDC_EVENT_DISPLAY_CONNECT notifies the LCDC that the
  display has been connected
- SH_MOBILE_LCDC_EVENT_DISPLAY_DISCONNECT notifies the LCDC that the
  display has been disconnected
- SH_MOBILE_LCDC_EVENT_DISPLAY_MODE notifies that LCDC that a display
  mode has been detected

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>