ARM: dts: omap3-gta04: declare backlight in lcd node

The lcd display of the gta04 has a backlight but the backlight
was not referenced in the lcd node, so screen blanking did
not turn off the backlight. Fix that.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>