mtd: spi-mem: 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>

drivers/spi/stm32_qspi.c

@@ -9,8 +9,10 @@
 
 #include <common.h>
 #include <clk.h>
+#include <dm.h>
 #include <log.h>
 #include <reset.h>
+#include <spi.h>
 #include <spi-mem.h>
 #include <dm/device_compat.h>
 #include <linux/bitops.h>