generic: ar8216: add support for ar8229

generic: ar8216: add support for ar8229

ar8229 is the builtin switch in ar934x and later chips. There is
also a standalone version available and their registers/functions
are the same.

This commit added support for the builtin ar8229. The only thing
missing for standalone ar8229 should be phy modes. Since I don't
have a router using that, this commit doesn't add support for
other phy modes.

Only add its support for mdio-device probing method because the
current PHY probing can't return 1G speed when it's a FE switch.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>