adc: Drop dm.h header file

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

Drop the common.h inclusion also.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/adc/stm32-adc.c b/drivers/adc/stm32-adc.c
index b12f894..3f0ed48 100644
--- a/drivers/adc/stm32-adc.c
+++ b/drivers/adc/stm32-adc.c
@@ -8,6 +8,7 @@
 
 #include <common.h>
 #include <adc.h>
+#include <dm.h>
 #include <asm/io.h>
 #include <dm/device_compat.h>
 #include <linux/bitops.h>