pwm_backlight: add device tree support for Low Threshold Brightness

pwm_backlight: add device tree support for Low Threshold Brightness

Some backlights perform poorly when driven by a PWM with a short
duty-cycle.  For such devices, the low threshold can be used to specify a
lower bound for the duty-cycle and should be chosen to exclude the
problematic range.

Add device tree probing support for
platform_pwm_backlight_data,lth_brightness, putting
low-threshold-brightness as the optional property's name.

Signed-off-by: Philip, Avinash <avinashphilip@ti.com>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>