net: phy: mscc: add support for VSC8501

The VSC8501 PHY can use the same driver implementation as the VSC8502.
Adding the PHY ID and copying the handler functions of VSC8502 is
sufficient to operate it.

Signed-off-by: David Epping <david.epping@missinglinkelectronics.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>