crypto: x86/aes-xts - change license to Apache-2.0 OR BSD-2-Clause

As with the other AES modes I've implemented, I've received interest in
my AES-XTS assembly code being reused in other projects.  Therefore,
change the license to Apache-2.0 OR BSD-2-Clause like what I used for
AES-GCM.  Apache-2.0 is the license of OpenSSL and BoringSSL.

Note that it is difficult to *directly* share code between the kernel,
OpenSSL, and BoringSSL for various reasons such as perlasm vs. plain
asm, Windows ABI support, different divisions of responsibility between
C and asm in each project, etc.  So whether that will happen instead of
just doing ports is still TBD.  But this dual license should at least
make it possible to port changes between the projects.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>