dt-bindings: net: wireless: mt76: document bindings for MT7622

MT7622 is a SoC that includes a 2.4 GHz 4x4 802.11n WMAC. Its
feature set is comparable to a MT7615 chip, but limited to 2.4 GHz.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>