net: phy: vitesse: Add support for VSC73xx

The VSC7385, VSC7388, VSC7395 and VSC7398 are integrated
switch/router chips for 5+1 or 8-port switches/routers. When
managed directly by Linux using DSA we need to do a special
set-up "dance" on the PHY. Unfortunately these sequences
switches the PHY to undocumented pages named 2a30 and 52b6
and does undocumented things. It is described by these opaque
sequences also in the reference manual. This is a best
effort to integrate it anyways.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>