UPSTREAM: crypto: chacha20 - Add a generic ChaCha20 stream cipher implementation

ChaCha20 is a high speed 256-bit key size stream cipher algorithm designed by
Daniel J. Bernstein. It is further specified in RFC7539 for use in IETF
protocols as a building block for the ChaCha20-Poly1305 AEAD.

This is a portable C implementation without any architecture specific
optimizations. It uses a 16-byte IV, which includes the 12-byte ChaCha20 nonce
prepended by the initial block counter. Some algorithms require an explicit
counter value, for example the mentioned AEAD construction.

Signed-off-by: Martin Willi <martin@strongswan.org>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
(cherry picked from commit c08d0e647305c3f8f640010a56c9e4bafb9488d3)
Change-Id: I5892b1451e46f915c0ed8e711bdded9e6f4a4aae
Signed-off-by: Eric Biggers <ebiggers@google.com>