dt-bindings:net:wireless:mediatek,mt76: add disable-radar-offchan

Add the capability to disable/enable radar/CAC detection running on
a dedicated offchannel chain available on some hw.
Offchannel radar/CAC detection allows to avoid CAC downtime switching
on a different channel during CAC detection on the selected radar
channel.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>