gpio: 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/gpio/README.md

@@ -63,3 +63,10 @@ use to communicate as well as share the guests memory over a memfd.
       -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on \
       -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/gpio/src/backend.rs

@@ -3,7 +3,7 @@
 // Copyright 2022 Linaro Ltd. All Rights Reserved.
 //          Viresh Kumar <viresh.kumar@linaro.org>
 //
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
 use log::{info, warn};
 use std::num::ParseIntError;

crates/gpio/src/gpio.rs

@@ -3,7 +3,7 @@
 // Copyright 2022 Linaro Ltd. All Rights Reserved.
 //          Viresh Kumar <viresh.kumar@linaro.org>
 //
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
 use log::error;
 use std::sync::{Arc, RwLock};

crates/gpio/src/main.rs

@@ -3,7 +3,7 @@
 // Copyright 2022 Linaro Ltd. All Rights Reserved.
 //          Viresh Kumar <viresh.kumar@linaro.org>
 //
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
 #[cfg(target_env = "gnu")]
 mod backend;

crates/gpio/src/vhu_gpio.rs

@@ -3,7 +3,7 @@
 // Copyright 2022 Linaro Ltd. All Rights Reserved.
 //          Viresh Kumar <viresh.kumar@linaro.org>
 //
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
 use log::error;
 use std::mem::size_of;