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.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>