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>

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c

@@ -408,6 +408,9 @@ stmmac_probe_config_dt(struct platform_device *pdev, const char **mac)
 	/* Default to phy auto-detection */
 	plat->phy_addr = -1;
 
+	/* Get clk_csr from device tree */
+	of_property_read_u32(np, "clk_csr", &plat->clk_csr);
+
 	/* "snps,phy-addr" is not a standard property. Mark it as deprecated
 	 * and warn of its use. Remove this when phy node support is added.
 	 */