dt-bindings: net: wireless: mt76: allow all 4 interrupts for MT7981

MT7981 (Filogic 820) is a low cost version of MT7986 (Filogic 830) with
a similar wireless controller that also supports four interrupts.

Cc: Peter Chiu <chui-hao.chiu@mediatek.com>
Cc: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>