ABI: testing: sysfs-class-oxp: add tt_led attribute documentation

ABI: testing: sysfs-class-oxp: add tt_led attribute documentation

Adds documentation about the tt_led attribute of OneXPlayer devices to
the sysfs-class-oxp ABI documentation.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Derek J. Clark <derekjohn.clark@gmail.com>
Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
Link: https://lore.kernel.org/r/20250425111821.88746-6-lkml@antheas.dev
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>