Alter grammar to allow multiple /dts-v1/ tags

This patch allows dtc to accept multiple /dts-v1/ tags (provided they're
all at the beginning of the input), rather than giving a syntax error.

This makes it more convenient to include one .dts file from another without
having to be careful that the /dts-v1/ tag is in exactly one of them.

We a couple of existing testcases to take advantage of this, which
simplifies them slightly.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc-parser.y

@@ -101,13 +101,18 @@ extern bool treesource_error;
 %%
 
 sourcefile:
-	  DT_V1 ';' memreserves devicetree
+	  v1tag memreserves devicetree
 		{
-			the_boot_info = build_boot_info($3, $4,
-							guess_boot_cpuid($4));
+			the_boot_info = build_boot_info($2, $3,
+							guess_boot_cpuid($3));
 		}
 	;
 
+v1tag:
+	  DT_V1 ';'
+	| DT_V1 ';' v1tag
+	;
+
 memreserves:
 	  /* empty */
 		{