rng: update License in source files and readme

Cargo.toml defines `license = "Apache-2.0 OR BSD-3-Clause"`,
so let's update the SPDX in source files and add a section
in the readme about the license.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

crates/rng/README.md

@@ -71,3 +71,10 @@ for the backend RNG device to communicate with the vhost RNG daemon:
       -device vhost-user-rng-pci,chardev=rng0                               \
       -numa node,memdev=mem                                                 \
       ...
+
+## License
+
+This project is licensed under either of
+
+- [Apache License](http://www.apache.org/licenses/LICENSE-2.0), Version 2.0
+- [BSD-3-Clause License](https://opensource.org/licenses/BSD-3-Clause)

crates/rng/src/main.rs

@@ -2,7 +2,7 @@
 // Copyright 2022 Linaro Ltd. All Rights Reserved.
 // Mathieu Poirier <mathieu.poirier@linaro.org>
 //
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 mod vhu_rng;
 
 use log::{info, warn};

crates/rng/src/vhu_rng.rs

@@ -3,7 +3,7 @@
 // Copyright 2022 Linaro Ltd. All Rights Reserved.
 // Mathieu Poirier <mathieu.poirier@linaro.org>
 //
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
 use log::warn;
 use std::io::Read;