leds: lp5562: Add multicolor brightness control

The framework for multicolor brightness control is already in place
in the lp55xx-common code but the function to control the multicolor
brightness for this particular chip is still missing.

Implement the multicolor_brightness_fn function to allow multicolor
brightness control of LEDs connected to the LP5562 LED driver.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Link: https://lore.kernel.org/r/20241009095635.2790613-1-michal.vokac@ysoft.com


Signed-off-by: Lee Jones <lee@kernel.org>