sandbox: spl: Lower priority of standard loader

We normally want to load U-Boot from SPL, but if a board wants to do
something else, it is currently not possible since the standard loader
has the top priority. Lower it to allow other SPL_LOAD_IMAGE_METHOD()
declarations to override it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/spl.c b/arch/sandbox/cpu/spl.c
index 2ca4cd6..106a78a 100644
--- a/arch/sandbox/cpu/spl.c
+++ b/arch/sandbox/cpu/spl.c
@@ -44,7 +44,7 @@
 
 	return 0;
 }
-SPL_LOAD_IMAGE_METHOD("sandbox", 0, BOOT_DEVICE_BOARD, spl_board_load_image);
+SPL_LOAD_IMAGE_METHOD("sandbox", 9, BOOT_DEVICE_BOARD, spl_board_load_image);
 
 void spl_board_init(void)
 {