etherswitch-adm6996fc atu support

From: Mori Hiroki <yamori813_at_yahoo.co.jp>
Date: Tue, 04 Apr 2023 01:46:45 UTC
Hi

I add atu support to adm6996fc.

https://github.com/ZRouter/freebsd-src/commit/4562a4244ae0e46ffcfc65e734ecf4df7efc5ac5

adm6996fc datasheet have not atu access information.

I do investigate it.

I ony check on 12-STABLE.

Thanks

Hiroki Mori