crypto: arm64/crct10dif - preparatory refactor for 8x8 PMULL version

Reorganize the CRC-T10DIF asm routine so we can easily instantiate an
alternative version based on 8x8 polynomial multiplication in a
subsequent patch.

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