dt: add helper for 64bit cell adds

Some times in the device tree, we find an array of 2 u32 cells that
really are a single u64 value. This patch adds a helper to make the
creation of these easy.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
diff --git a/device_tree.c b/device_tree.c
index 7541274..c8d68c2 100644
--- a/device_tree.c
+++ b/device_tree.c
@@ -154,6 +154,13 @@
     return r;
 }
 
+int qemu_devtree_setprop_u64(void *fdt, const char *node_path,
+                             const char *property, uint64_t val)
+{
+    val = cpu_to_be64(val);
+    return qemu_devtree_setprop(fdt, node_path, property, &val, sizeof(val));
+}
+
 int qemu_devtree_setprop_string(void *fdt, const char *node_path,
                                 const char *property, const char *string)
 {