Documentation: networking: phy.txt: MDIO bus reset is optional

Update the MDIO bus documentation to mention that the MDIO bus reset
function is completely optional. It became optional with commit
e1393456

 ("[PATCH] PHY Layer fixup")

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