expo: Create a struct for generic text attributes

In preparation for adding more text types, refactor the common fields
into a new structure. This will allow common code to be used.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/boot/bootflow.c b/test/boot/bootflow.c
index 58885b5..ea1a994 100644
--- a/test/boot/bootflow.c
+++ b/test/boot/bootflow.c
@@ -858,7 +858,7 @@
 	txt = scene_obj_find(scn, id, SCENEOBJT_TEXT);
 	ut_assertnonnull(txt);
 
-	ut_asserteq(font_size, txt->font_size);
+	ut_asserteq(font_size, txt->gen.font_size);
 
 	return 0;
 }