crypto: arm64 - implement SHA-512 using special instructions

Implement the SHA-512 using the new special instructions that have
been introduced as an optional extension in ARMv8.2.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>