FROMGIT: crypto: arm/chacha20 - refactor to allow varying number of rounds

In preparation for adding XChaCha12 support, rename/refactor the NEON
implementation of ChaCha20 to support different numbers of rounds.

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit 3cc215198eac75cc4130729ddd94a5cdbdb4d300
 https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git

 master)
Bug: 112008522
Test: As series, see Ic61c13b53facfd2173065be715a7ee5f3af8760b
Change-Id: Iba4b7ea5ecee72c28b7e4188df438335d0d5b9aa
Signed-off-by: Eric Biggers <ebiggers@google.com>