[arge] add a comment about needing mdio busses in order to use the interface.

This is a holdover from how reset is handled in the ARGE_MDIO world.
You need to define the mdio bus device if you want to use the ethernet
device or the arge setup path doesn't bring the MAC out of reset.

sys/mips/atheros/if_arge.c

@@ -885,6 +885,11 @@ arge_attach(device_t dev)
 	/*
 	 * Don't do this for the MDIO bus case - it's already done
 	 * as part of the MDIO bus attachment.
+	 *
+	 * XXX TODO: if we don't do this, we don't ever release the MAC
+	 * from reset and we can't use the port.  Now, if we define ARGE_MDIO
+	 * but we /don't/ define two MDIO busses, then we can't actually
+	 * use both MACs.
 	 */
 #if !defined(ARGE_MDIO)
 	/* Initialize the MAC block */