Add driver for Allwinner A83T/H3/A64 Gigabit Ethernet.

The datasheets refer to this controller as EMAC, not to be confused with
the fast ethernet controller (also named EMAC) found in A10/A20 SoCs.

Tested on a BananaPi M3 (A83T), which uses an external RGMII PHY (RTL8211E).

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D6169