crypto: arm64/chacha20 - implement NEON version based on SSE3 code

This is a straight port to arm64/NEON of the x86 SSE3 implementation
of the ChaCha20 stream cipher.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>