video: lm3530_bl: Introduce new tunables to control LED driver
- min_br: sets the mininum brightness level (2-114)
- max_br: sets the maximum brightness level (2-114)
- br_mode: sets the linear (default 1) or exponential (0) brightness
mapping mode.

Credits to @Stratos Karafotis
lm3530: Add a toggle for the CABC PWM

Conflicts:
	drivers/video/backlight/lm3530_bl.c