net: Drop dm.h header file from phy.h

This header file should not be included in other header files. Remove it
and use other headers and C inclusions instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/mdio.c b/cmd/mdio.c
index c48bb51..cfa45ad 100644
--- a/cmd/mdio.c
+++ b/cmd/mdio.c
@@ -10,6 +10,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <dm.h>
 #include <miiphy.h>
 #include <phy.h>