net: ethernet: stmmac: add management of clk_csr property

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: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>