crypto: arm64/sha256-neon - play nice with CONFIG_PREEMPT kernels

Tweak the SHA256 update routines to invoke the SHA256 block transform
block by block, to avoid excessive scheduling delays caused by the
NEON algorithm running with preemption disabled.

Also, remove a stale comment which no longer applies now that kernel
mode NEON is actually disallowed in some contexts.

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