ARM: dts: am437x-sk-evm: Add phandle for the backlight for the panel

With the backlight phandle the driver can manage the backlight on/off in
sync with the panel enable/disable.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>