[etherswitch] add infineon adm6996fc support on etherswitch

This is Infineon ADM6996FC/M/MX driver code on etherswitch framework.
Support PORT and DOT1Q VLAN.
This code suppose ADM6996FC SDC/SDIO connect to SOC network interface
MDC/MDIO.
This code tested on Netgear WGR614Cv7.

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: adrian, mizhka
Approved by: adrian(mentor)
Differential Revision: https://reviews.freebsd.org/D8495