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>
diff --git a/common/spl/spl_mmc.c b/common/spl/spl_mmc.c
index 97c11b3..899caf4 100644
--- a/common/spl/spl_mmc.c
+++ b/common/spl/spl_mmc.c
@@ -267,14 +267,14 @@
 }
 #endif
 
-int spl_mmc_load_image(u32 boot_device)
+int spl_mmc_load_image(struct spl_boot_device *bootdev)
 {
 	struct mmc *mmc = NULL;
 	u32 boot_mode;
 	int err = 0;
 	__maybe_unused int part;
 
-	err = spl_mmc_find_device(&mmc, boot_device);
+	err = spl_mmc_find_device(&mmc, bootdev->boot_device);
 	if (err)
 		return err;
 
@@ -286,7 +286,7 @@
 		return err;
 	}
 
-	boot_mode = spl_boot_mode(boot_device);
+	boot_mode = spl_boot_mode(bootdev->boot_device);
 	err = -EINVAL;
 	switch (boot_mode) {
 	case MMCSD_MODE_EMMCBOOT: