net: introduce MDIO DM class for MDIO devices

Adds UCLASS_MDIO DM class supporting MDIO buses that are probed as
stand-alone devices.  Useful in particular for systems that support
DM_ETH and have a stand-alone MDIO hardware block shared by multiple
Ethernet interfaces.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

cmd/mdio.c

@@ -203,6 +203,11 @@ static int do_mdio(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	if (argc < 2)
 		return CMD_RET_USAGE;
 
+#ifdef CONFIG_DM_MDIO
+	/* probe DM MII device before any operation so they are all accesible */
+	dm_mdio_probe_devices();
+#endif
+
 	/*
 	 * We use the last specified parameters, unless new ones are
 	 * entered.