Introduce basic etherswitch support for Ralink SoCs

This revision introduces basic support for the internal ESW switch found
Ralink/Mediatek SoCs such as RT3050, RT3352, RT5350, MT7628; and GSW
found in MT7620 and MT7621.

It only supports 802.1q VLANs and doesn't support external PHYs at the
moment (only the ones that are built into the switch itself).

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D6348