sf: Drop dm.h header file from spi_flash.h

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>

include/spi_flash.h

@@ -9,10 +9,11 @@
 #ifndef _SPI_FLASH_H_
 #define _SPI_FLASH_H_
 
-#include <dm.h>	/* Because we dereference struct udevice here */
 #include <linux/types.h>
 #include <linux/mtd/spi-nor.h>
 
+struct udevice;
+
 /* by default ENV use the same parameters than SF command */
 #ifndef CONFIG_ENV_SPI_BUS
 # define CONFIG_ENV_SPI_BUS	CONFIG_SF_DEFAULT_BUS