spl: MMC U-Boot image load from raw partition

Raw images of U-Boot can be stored inside MMC partitions, so it makes sense to
read the partition table, looking for a partition number instead of using
a fixed sector address.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@ti.com>
[trini: Only add mmc_load_image_raw_partition() when
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION to avoid warning, add missing
conversion in spl_mmc_load_image()]
Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/README b/README
index f827546..bfefa21 100644
--- a/README
+++ b/README
@@ -3660,6 +3660,10 @@
 		Address and partition on the MMC to load U-Boot from
 		when the MMC is being used in raw mode.
 
+		CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION
+		Partition on the MMC to load U-Boot from when the MMC is being
+		used in raw mode
+
 		CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR
 		Sector to load kernel uImage from when MMC is being
 		used in raw mode (for Falcon mode)