dtc: Implement checks for the format of node and property names

This patch adds checks to the checking framework to verify that node
and property names contain only legal characters, and in the case of
node names there is at most one '@'.

At present when coming from dts input, this is mostly already ensured
by the grammer, however putting the check later means its easier to
generate helpful error messages rather than just "syntax error".  For
dtb input, these checks replace the older similar check built into
flattree.c.

Testcases for the checks are also implemented.

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

tests/dumptrees.c

@@ -37,6 +37,7 @@ struct {
 } trees[] = {
 #define TREE(name)	{ &_##name, #name ".dtb" }
 	TREE(test_tree1),
+	TREE(bad_node_char), TREE(bad_node_format), TREE(bad_prop_char),
 };
 
 #define NUM_TREES	(sizeof(trees) / sizeof(trees[0]))