mscc: Drop dm.h header file

This header file should not be included in other header files. Remove it
from each one and use a forward declaration instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/mscc/servalt/servalt.c

@@ -9,6 +9,8 @@
 #include <asm/io.h>
 #include <led.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
 enum {
 	BOARD_TYPE_PCB116 = 0xAABBCE00,
 };