This revision adds support to if_rt for more SoCs.

The SoCs I've tried the driver with include the following:
RT3050, RT5350, RT3662, RT3883, MT7620, MT7621, MT7688.
On boards, based on the above SoCs traffic is passing through correctly
and the boards survive a flood ping with very little or no drops (drops
may be caused elsewhere in my test setup, however).

One issue still remains and needs to be fixed in the future: if_rt does
not survive an ifconfig rt0 down/ifconfig rt0 up cycle.
This issue existed before this commit as well, however.

Reviewed by: ray
Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5864