dt-bindings: leds: Document "rc-feedback" trigger

Document the "rc-feedback" trigger which is used to control LEDs by
remote control device activity. This is an existing trigger used in
existing DTs, document it so validation of those DTs would pass.

It was originally introduced into the Linux kernel in 2013 with
commit 153a60bb ("[media] rc: add feedback led trigger for rc keypresses")

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241007205315.2477060-1-heiko@sntech.de


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