spl: mmc: Introduce proper layering for spl_mmc_get_uboot_raw_sector()

Introduce two new weak functions, arch_spl_mmc_get_uboot_raw_sector() and
board_spl_mmc_get_uboot_raw_sector(), each of which can be overridden at
a matching level, that is arch/ and board/ , in addition to the existing
weak function spl_mmc_get_uboot_raw_sector().

This way, architecture code can define a default architecture specific
implementation of arch_spl_mmc_get_uboot_raw_sector(), while the board
code can override that using board_spl_mmc_get_uboot_raw_sector() which
takes precedence over the architecture code. In some sort of unlikely
special case where code has to take precedence over board code too, the
spl_mmc_get_uboot_raw_sector() is still left out to be a weak function,
but it should be unlikely that this is ever needed to be overridden.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/board/data_modul/imx8mp_edm_sbc/spl.c b/board/data_modul/imx8mp_edm_sbc/spl.c
index 2fdd95a..cfc4b65 100644
--- a/board/data_modul/imx8mp_edm_sbc/spl.c
+++ b/board/data_modul/imx8mp_edm_sbc/spl.c
@@ -107,7 +107,7 @@
 	spl_boot_list[4] = BOOT_DEVICE_NONE;
 }
 
-unsigned long spl_mmc_get_uboot_raw_sector(struct mmc *mmc, unsigned long sect)
+unsigned long board_spl_mmc_get_uboot_raw_sector(struct mmc *mmc, unsigned long sect)
 {
 	const u32 boot_dev = spl_boot_device();
 	int part;