leds: lp5523: add master_fader support

This patch introduces 4 new attributes:
  master_fader_leds
  master_fader1
  master_fader2
  master_fader3

Fo example, to map channel 0,6 to master_fader1,
map channel 1,7 to master_fader2,
map channel 2,8 to master_fader3, and
map channel 3,4,5 to none

  echo "123000123" > master_fader_leds

A different factor can be set to each master_fader:

  echo 255 > master_fader1
  echo 100 > master_fader2
  echo 0 > master_fader3

Signed-off-by: Toshi Kikuchi <toshik@chromium.org>
Acked-by: Milo Kim <milo.kim@ti.com>
Tested-by: Milo Kim <milo.kim@ti.com>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>