dtoc: Add support for reading 64-bit ints

Add functions to read a 64-bit integer property from the devicetree.

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 5a6fa88..4c2c70a 100644
--- a/tools/dtoc/test/dtoc_test_simple.dts
+++ b/tools/dtoc/test/dtoc_test_simple.dts
@@ -16,6 +16,7 @@
 		boolval;
 		maybe-empty-int = <>;
 		intval = <1>;
+		int64val = /bits/ 64 <0x123456789abcdef0>;
 		intarray = <2 3 4>;
 		byteval = [05];
 		bytearray = [06];