OMAPDSS: panels: add Kconfig comment

Add a comment to Kconfig to clarify the difference between the two
display driver directories.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/displays-new/Kconfig

-menu "OMAP Display Device Drivers"
+menu "OMAP Display Device Drivers (new device model)"
         depends on OMAP2_DSS
 
 config DISPLAY_ENCODER_TFP410

drivers/video/omap2/displays/Kconfig

-menu "OMAP2/3 Display Device Drivers"
+menu "OMAP2/3 Display Device Drivers (old device model)"
         depends on OMAP2_DSS
 
 config PANEL_GENERIC_DPI