Fix test failure caused by bad handling of ramdisk

Commit e3a5bbce broke the FIT image tests by not loading a ramdisk even if
a load address is provided in the FIT. The rationale was that a load address
of 0 should be considered to mean 'do not load'.

Add a new load operation which supports this feature, so that the ramdisk
will be loaded if a non-zero load address is provided.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/image-fit.c b/common/image-fit.c
index c61be65..255c4ca 100644
--- a/common/image-fit.c
+++ b/common/image-fit.c
@@ -1656,7 +1656,7 @@
 			bootstage_error(bootstage_id + BOOTSTAGE_SUB_LOAD);
 			return -EBADF;
 		}
-	} else {
+	} else if (load_op != FIT_LOAD_OPTIONAL_NON_ZERO || load) {
 		ulong image_start, image_end;
 		ulong load_end;
 		void *dst;