ARM: dts: omap4-droid4: Add LCD backlight

The TI LMU driver has not yet been merged, but the device
tree binding for TI LMU drivers has been acked already
earlier by Rob Herring <robh+dt@kernel.org>. So it should
be safe to apply to cut down the number of pending patches.

Cc: devicetree@vger.kernel.org
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Acked-by: Milo Kim <milo.kim@ti.com>
Tested-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>