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>

common/spl/spl_sata.c

@@ -20,7 +20,7 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
-int spl_sata_load_image(void)
+int spl_sata_load_image(struct spl_boot_device *bootdev)
 {
 	int err;
 	struct blk_desc *stor_dev;