spl: Convert boot_device into a struct

At present some spl_xxx_load_image() functions take a parameter and some
don't. Of those that do, most take an integer but one takes a string.

Convert this parameter into a struct so that we can pass all functions the
same thing. This will allow us to use a common function signature.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

drivers/mtd/spi/spi_spl_load.c

@@ -65,7 +65,7 @@ static ulong spl_spi_fit_read(struct spl_load_info *load, ulong sector,
  * configured and available since this code loads the main U-Boot image
  * from SPI into SDRAM and starts it from there.
  */
-int spl_spi_load_image(void)
+int spl_spi_load_image(struct spl_boot_device *bootdev)
 {
 	int err = 0;
 	struct spi_flash *flash;

drivers/mtd/spi/sunxi_spi_spl.c

@@ -262,7 +262,7 @@ static void spi0_read_data(void *buf, u32 addr, u32 len)
 
 /*****************************************************************************/
 
-int spl_spi_load_image(void)
+int spl_spi_load_image(struct spl_boot_device *bootdev)
 {
 	int err;
 	struct image_header *header;