kernel: 5.15: Backport Turris Omnia LED patches

kernel: 5.15: Backport Turris Omnia LED patches

This backports patches
  leds: turris-omnia: convert to use dev_groups
  leds: turris-omnia: Use sysfs_emit() instead of sprintf()
  leds: turris-omnia: Drop unnecessary mutex locking
  leds: turris-omnia: Do not use SMBUS calls
  leds: turris-omnia: Make set_brightness() more efficient
  leds: turris-omnia: Support HW controlled mode via private trigger
  leds: turris-omnia: Add support for enabling/disabling HW gamma correction
  leds: turris-omnia: Fix brightness setting and trigger activating
into backport-5.15.

The above patches replace:
  leds: turris-omnia: support HW controlled mode via private trigger
  leds: turris-omnia: initialize multi-intensity to full
  leds: turris-omnia: change max brightness from 255 to 1
from mvebu/patches-5.15.

Signed-off-by: Marek Behún <kabel@kernel.org>