dt-bindings: net: wireless: mt76: Document power-limits country property

The commit 22b980badc0f ("mt76: add functions for parsing rate power limits
from DT") added filtering of the power limits based on two properties:

* regdomain
* country

If either the country or the regdomain matches, the power limits are
applied and the search is aborted. If none of the two is defined for the
power limit, it is a global (or "fallback") power limit. The last
"fallback" power limit in the list will be returned when not matching
regdomain or country was found.

The idea is here to allow to specify "overwriting" country limits in front
of the list - just in case a regdomain is shared but a country has
additional limitations.

But this property was forgotten to be defined in commit 2de6ccebe0e7
("dt-bindings:net:wireless:mediatek,mt76: introduce power-limits node").

Signed-off-by: Sven Eckelmann (Plasma Cloud) <se@simonwunderlich.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251007-backoff-table-support-v3-1-fd6e2684988f@simonwunderlich.de
Signed-off-by: Felix Fietkau <nbd@nbd.name>