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>