net/macb: Add arch specific routine to get mdio control

SPEAr310 and SPEAr320 Ethernet interfaces share same MDIO lines to control their
respective phys. Currently there is a fixed configuration in which only a
particular MAC can use the MDIO lines.

Call an arch specific function to take control of specific mdio lines at
runtime.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Acked-by: Stefan Roese <sr@denx.de>