net: phy: vitesse: add support for VSC8601

This adds support for the Vitesse VSC8601 PHY. Generic functions are
used for everything except interrupt handling.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>