spl: mmc: Factor out eMMC boot partition selection code

Factor out eMMC boot partition selection code into
default_spl_mmc_emmc_boot_partition() function and implement
weak spl_mmc_emmc_boot_partition(), so that architecture or
board code can override the eMMC boot partition selection.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Faiz Abbas <faiz_abbas@ti.com>
Cc: Harald Seiler <hws@denx.de>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/include/spl.h b/include/spl.h
index cee9a42..c643943 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -357,6 +357,29 @@
  * If not overridden, it is weakly defined in common/spl/spl_mmc.c.
  */
 int spl_mmc_boot_partition(const u32 boot_device);
+
+struct mmc;
+/**
+ * default_spl_mmc_emmc_boot_partition() - eMMC boot partition to load U-Boot from.
+ * mmc:			Pointer for the mmc device structure
+ *
+ * This function should return the eMMC boot partition number which
+ * the SPL should load U-Boot from (on the given boot_device).
+ */
+int default_spl_mmc_emmc_boot_partition(struct mmc *mmc);
+
+/**
+ * spl_mmc_emmc_boot_partition() - eMMC boot partition to load U-Boot from.
+ * mmc:			Pointer for the mmc device structure
+ *
+ * This function should return the eMMC boot partition number which
+ * the SPL should load U-Boot from (on the given boot_device).
+ *
+ * If not overridden, it is weakly defined in common/spl/spl_mmc.c
+ * and calls default_spl_mmc_emmc_boot_partition();
+ */
+int spl_mmc_emmc_boot_partition(struct mmc *mmc);
+
 void spl_set_bd(void);
 
 /**