leds: turris-omnia: Add support for enabling/disabling HW gamma correction

If the MCU on Turris Omnia is running newer firmware versions, the LED
controller supports RGB gamma correction (and enables it by default for
newer boards).

Determine whether the gamma correction setting feature is supported and
add the ability to set it via sysfs attribute file.

Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20230918161104.20860-5-kabel@kernel.org


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