Documentation: leds-lp5523: describe master fader attributes

2) Firmware interface - LP55xx common interface

  For the details, please refer to 'firmware' section in leds-lp55xx.txt

LP5523 has three master faders. If a channel is mapped to one of

the master faders, its output is dimmed based on the value of the master

fader.

For example,

  echo "123000123" > master_fader_leds

creates the following channel-fader mappings:

  channel 0,6 to master_fader1

  channel 1,7 to master_fader2

  channel 2,8 to master_fader3

Then, to have 25% of the original output on channel 0,6:

  echo 64 > master_fader1

To have 0% of the original output (i.e. no output) channel 1,7:

  echo 0 > master_fader2

To have 100% of the original output (i.e. no dimming) on channel 2,8:

  echo 255 > master_fader3

To clear all master fader controls:

  echo "000000000" > master_fader_leds

Selftest uses always the current from the platform data.

Each channel contains led current settings.