spl: Add function prototype for spl_mmc_get_uboot_raw_sector

We did not add a prototype for spl_mmc_get_uboot_raw_sector to
include/spl.h before, so add and document one now. Correct the incorrect
prototype in board/advantech/imx8mp_rsb3720a1/imx8mp_rsb3720a1.c and
ensure that we have spl.h where we define a non-weak
spl_mmc_get_uboot_raw_sector as well.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/advantech/imx8mp_rsb3720a1/imx8mp_rsb3720a1.c b/board/advantech/imx8mp_rsb3720a1/imx8mp_rsb3720a1.c
index 34109c6..09e63e0 100644
--- a/board/advantech/imx8mp_rsb3720a1/imx8mp_rsb3720a1.c
+++ b/board/advantech/imx8mp_rsb3720a1/imx8mp_rsb3720a1.c
@@ -208,7 +208,8 @@
 
 #ifdef CONFIG_SPL_MMC
 #define UBOOT_RAW_SECTOR_OFFSET 0x40
-unsigned long spl_mmc_get_uboot_raw_sector(struct mmc *mmc)
+unsigned long spl_mmc_get_uboot_raw_sector(struct mmc *mmc,
+					   unsigned long raw_sector)
 {
 	u32 boot_dev = spl_boot_device();