mt76: import mediatek/mt76 driver

Import ISC-licensed driver parts of mediatek/mt76
assumed to be based on Linux wireless-testing at
a02411a5b98612c12be99349836d99f07db12a77 (tag: wt-2022-11-23).

Complement the driver and LinuxKPI with our own (dummy)
implementations of missing parts (util.h and soc/mediatek/)
as well as changes to make compile on FreeBSD with changes
covered by #ifdef (FreeBSD) conditions.
Further select updates were applied since the initial import
in order to keep compiling along with other LinuxKPI based
drivers.

For the moment we only target the mt7915 and mt7921 PCI parts.
More may follow in the future.

Firmware is provided by port net/wifi-firmware-mt76-kmod.

Given the lack of full license texts on non-local files this is
imported under the draft policy for handling SPDX files (D29226). [1]

Approved by: core (emaste, 2022-04-08) [1]
MFC after: 2 months