spl: blk: Support loading images from fs

Add a generic API to support loading of SPL payload from any supported
filesystem on a given partition of a block device.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
diff --git a/include/spl.h b/include/spl.h
index 7e0f5ac..20e1eb3 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -672,6 +672,9 @@
 int spl_load_image_ext_os(struct spl_image_info *spl_image,
 			  struct spl_boot_device *bootdev,
 			  struct blk_desc *block_dev, int partition);
+int spl_blk_load_image(struct spl_image_info *spl_image,
+		       struct spl_boot_device *bootdev,
+		       enum uclass_id uclass_id, int devnum, int partnum);
 
 /**
  * spl_early_init() - Set up device tree and driver model in SPL if enabled