dtoc: Support reading a list of arguments

It is helpful to support a string or stringlist containing a list of
space-separated arguments, for example:

   args = "-n fred", "-a", "123";

This resolves to the list:

   -n fred -a 123

which can be passed to a program as arguments.

Add a helper to do the required processing.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test/dtoc_test_simple.dts b/tools/dtoc/test/dtoc_test_simple.dts
index 4c2c70a..2d321fb 100644
--- a/tools/dtoc/test/dtoc_test_simple.dts
+++ b/tools/dtoc/test/dtoc_test_simple.dts
@@ -62,5 +62,6 @@
 
 	orig-node {
 		orig = <1 23 4>;
+		args = "-n first", "second", "-p", "123,456", "-x";
 	};
 };