x86/64: Add support for AMD SOC Ethernet controller

x86/64: Add support for AMD SOC Ethernet controller

This feature is already supported in kernel. Add definition
and configuration in Makefile of OpenWRT.
It is tested on board Dibbler with processor V1605B.
Dmesg is as below.
--------------------------------------------
[    0.317122] smpboot: CPU0: AMD Ryzen Embedded V1605B with Radeon
Vega Gfx (family: 0x17, model: 0x11, stepping: 0x0)

[    8.227270] libphy: amd-xgbe-mii: probed
[    8.228189] amd-xgbe 0000:02:00.1 eth0: net device enabled
[    8.228999] libphy: amd-xgbe-mii: probed
[    8.229921] amd-xgbe 0000:02:00.2 eth1: net device enabled
.....
[   11.760869] br-lan: port 1(eth0) entered blocking state
[   11.761365] br-lan: port 1(eth0) entered disabled state
[   11.762019] device eth0 entered promiscuous mode
[   11.785285] 8021q: adding VLAN 0 to HW filter on device eth1
[   16.881431] amd-xgbe 0000:02:00.1 eth0: Link is Up - 1Gbps/Full -
flow control off
[   16.882108] br-lan: port 1(eth0) entered blocking state
[   16.882583] br-lan: port 1(eth0) entered forwarding state
[   16.883249] IPv6: ADDRCONF(NETDEV_CHANGE): br-lan: link becomes ready
-----------------------------------------------

Signed-off-by: Zheng Bao <fishbaozi@gmail.com>