BACKPORT: crypto: chacha20 - Export common ChaCha20 helpers

As architecture specific drivers need a software fallback, export a
ChaCha20 en-/decryption function together with some helpers in a header
file.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
(cherry picked from commit 31d7247da57226e847f0f102a10c27c0722c429b,
 omitted chacha20poly1305.c changes)
Change-Id: I044f18bf5b01f10da47ce17d58c3ecd4da941dba
Signed-off-by: Eric Biggers <ebiggers@google.com>