imx8m spl: load the M4 code from the boot partition

For eMMC booting from one of their hardware boot partitions, load the M4
binary from the hwpartition the SPL booted from.

This enables booting U-boot entirely from the eMMC boot1 or boot2
hwpartitions, thus keeping the bootloader and all the SoC-specific boot
black magic out of the way of the user.

board/emcraft/imx8m_som/ddr/lpddr4/helper.c

@@ -278,10 +278,13 @@ static struct mmc *prepare_mmc_boot_device(void)
 	struct mmc *mmc;
 	int err;
 	int boot_device;
+	int boot_mode;
+	__maybe_unused int part;
 
 	printf("Finding MMC device\n");
 
 	boot_device = spl_boot_device();
+	boot_mode = spl_boot_mode(boot_device);
 
 	err = spl_mmc_find_device(&mmc, boot_device);
 	if (err) {
@@ -294,6 +297,26 @@ static struct mmc *prepare_mmc_boot_device(void)
 		printf("spl: mmc init failed with error: %d\n", err);
 		return NULL;
 	}
+
+	/* Switch to the same partition as where the SPL is stored */
+	if (boot_mode == MMCSD_MODE_EMMCBOOT) {
+		int part;
+		part = (mmc->part_config >> 3) & PART_ACCESS_MASK;
+		if (part == 7)
+			part = 0;
+
+		printf("spl: switching to mmc hwpartition %d\n", part);
+		if (CONFIG_IS_ENABLED(MMC_TINY)) {
+			err = mmc_switch_part(mmc, part);
+		} else {
+			err = blk_dselect_hwpart(mmc_get_blk_desc(mmc), part);
+		}
+
+		if (err) {
+			printf("spl: mmc partition switch failed\n");
+			return NULL;
+		}
+	}
 	return mmc;
 }