In Documentation stmmac.txt there is possibility to
fixed CSR Clock range selection with property clk_csr.
This patch add the management of this property
For example to use it, add in your ethernet node DT:
	clk_csr = <3>;
Signed-off-by: NChristophe Roullier <christophe.roullier@st.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>