spl: Convert spl_nand_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/spl_nand.c b/common/spl/spl_nand.c
index 575de66..ed758e5 100644
--- a/common/spl/spl_nand.c
+++ b/common/spl/spl_nand.c
@@ -67,7 +67,7 @@
 	}
 }
 
-int spl_nand_load_image(void)
+static int spl_nand_load_image(struct spl_boot_device *bootdev)
 {
 	int err;
 	struct image_header *header;
@@ -145,3 +145,5 @@
 	return err;
 }
 #endif
+/* Use priorty 1 so that Ubi can override this */
+SPL_LOAD_IMAGE_METHOD(1, BOOT_DEVICE_NAND, spl_nand_load_image);