stmmac: add CSR Clock range selection

This patch adds the CSR Clock range selection.

Original patch from Johannes Stezenbach fixed the CSR
in the stmmac_mdio. We agreed to provide this through
the platform instead of.
Also thanks to Johannes for having tested it on ARM.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: David S. Miller <davem@davemloft.net>