stmmac: Allow stmmac to work with other PHY buses(v3).

As stmmac mdio bus name prefix is hardcoded in the driver, this allows
only phys on stmmac mdio buses to connect, however stmmac should allow
phys on other mdio buses too.

This patch adds new variable phy_bus_name to plat_stmmacenet_data
struct to let the BSP decide which phy bus to be used by stmmac driver.
A typical use-case is to have generic MDIO buses like mdio-gpio on top
of stmmac.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>