drivers/video/backlight/pwm_bl.c: add device tree support for Low Threshold Brightness

drivers/video/backlight/pwm_bl.c: add device tree support for Low Threshold Brightness

Low Threshold Brightness should be configured to have a linear relation in
brightness scale.  This patch adds device tree support for low threshold
brightness as optional one for pwm_backlight.

Signed-off-by: Philip, Avinash <avinashphilip@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>