wifi: mt76: mt7915: add support for MT7981

Add support for the MediaTek MT7981 SoC which is similar to the MT7986
but with a newer IP cores and only 2x ARM Cortex-A53 instead of 4x.
Unlike MT7986 the MT7981 can only connect a single wireless frontend,
usually MT7976 is used for DBDC.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>