thermal: Drop dm.h header file

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

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

arch/arm/mach-imx/cpu.c

@@ -8,6 +8,7 @@
 
 #include <bootm.h>
 #include <common.h>
+#include <dm.h>
 #include <init.h>
 #include <log.h>
 #include <net.h>

include/thermal.h

@@ -7,7 +7,7 @@
 #ifndef _THERMAL_H_
 #define _THERMAL_H_
 
-#include <dm.h>
+struct udevice;
 
 int thermal_get_temp(struct udevice *dev, int *temp);