crypto: lib/sha256 - Add helpers for block-based shash

Add an internal sha256_finup helper and move the finalisation code
from __sha256_final into it.

Also add sha256_choose_blocks and CRYPTO_ARCH_HAVE_LIB_SHA256_SIMD
so that the Crypto API can use the SIMD block function unconditionally.
The Crypto API must not be used in hard IRQs and there is no reason
to have a fallback path for hardirqs.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>