The beeper is currently not fully functional and has also
been removed from DTS.
Also remove the dependency for the gpio-beeper module.
Fixes: 695a1cd53c ("ath79: add support for MikroTik RouterBOARD 912UAG-2HPnD")
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>