dt-bindings: leds: Add FUNCTION defines for per-band WLANs

Most wireless routers and access points can operate in multiple bands
simultaneously. Vendors often equip their devices with per-band LEDs.

Add defines for those very common functions to allow cleaner & clearer
bindings.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240117151736.27440-1-zajec5@gmail.com


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