binman: Allow text directly in the node

At present text entries use an indirect method to specify the text to use,
with a label pointing to the text itself.

Allow the text to be directly written into the node. This is more
convenient in cases where the text is constant.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/test/066_text.dts

@@ -24,5 +24,10 @@
 			text-label = "test-id4";
 			test-id4 = "some text";
 		};
+		/* Put text directly in the node */
+		text5 {
+			type = "text";
+			text = "more text";
+		};
 	};
 };