With the LED backlight changes merged, we still need the dts configured
to have backlight working for droid4. Based on an earlier patch from
Pavel Machek <pavel@ucw.cz>, let's configure the backlight but update
the value range to be more usable.

We have a range of 256 register values split into 8 steps, so we can
generate the brightness levels backwards with:

$ for i in 0 1 2 3 4 5 6 7; do echo "255 - ${i} * (256 / 8)" | bc; done

To avoid more confusion why the LCD backlight is still not on, let's
also enable LED backlight as a loadable module for omap2plus_defconfig.

Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>