crypto: arm64/sha2-ce - add non-SIMD scalar fallback

The arm64 kernel will shortly disallow nested kernel mode NEON, so
add a fallback to scalar code that can be invoked in that case.

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