net: phy: vitesse: add support for VSC8572

Add support for the Vitesse VSC8572 which is functionally equivalent to
the already supported VSC8574. As such, all the same handling functions
are used since the VSC8572 merely has half the number of phy blocks
internally.

Signed-off-by: Stephen Agate <stephen.agate@uk.thalesgroup.com>
Signed-off-by: Neill Whillans <neill.whillans@codethink.co.uk>
Reviewed-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>