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>
diff --git a/arch/arm/mach-imx/cpu.c b/arch/arm/mach-imx/cpu.c
index 694c26d..fe8d594 100644
--- a/arch/arm/mach-imx/cpu.c
+++ b/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>