mtd: spi: Include dm.h in spi-nor-core.c

This header is needed so struct udevice can be used in dev_xxx().
Signed-off-by: NSean Anderson <seanga2@gmail.com>
Tested-by: NPatrick Delaunay <patrick.delaunay@st.com>

drivers/mtd/spi/spi-nor-core.c

@@ -11,6 +11,7 @@
 
 #include <common.h>
 #include <log.h>
+#include <dm.h>
 #include <dm/device_compat.h>
 #include <dm/devres.h>
 #include <linux/bitops.h>