linux-phytec-ti

backlight: add led-backlight driver

This patch adds a led-backlight driver (led_bl), which is similar to
pwm_bl except the driver uses a LED class driver to adjust the
brightness in the HW.

This patch has been discussed in mainline, but there was no conclusion
on the DT bindings:

http://www.spinics.net/lists/linux-leds/msg04937.html

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>