leds: lm355x, lm3642: support camera LED triggers for flash and torch

LM355x and LM3642 support flash and torch functionality.

 (Camera driver)          (LED trigger for camera)       (LED driver)
 Turn on the flash  ...>  ledtrig_flash_ctrl(true) ...>  LM355x or LM3642
                                                         brightness ctrl
                                                       <actual driving here>

Flash/torch LEDs are controlled by other driver using LED camera trigger
APIs, ledtrig_flash_ctrl()/ledtrig_torch_ctrl().
Then, actual device control is activated by each LED driver such like
LM355x or LM3642.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>