dtoc: Use None to mean stdout

At present dtoc uses '-' internally to mean that output should go to
stdout. This is not necessary and None is more convenient. Update it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/main.py b/tools/dtoc/main.py
index b94d9c3..7686c87 100755
--- a/tools/dtoc/main.py
+++ b/tools/dtoc/main.py
@@ -91,7 +91,7 @@
                   help='Specify the .dtb input file')
 parser.add_option('--include-disabled', action='store_true',
                   help='Include disabled nodes')
-parser.add_option('-o', '--output', action='store', default='-',
+parser.add_option('-o', '--output', action='store',
                   help='Select output filename')
 parser.add_option('-P', '--processes', type=int,
                   help='set number of processes to use for running tests')