armv8 SHA-256 using ARMv8 Crypto Extensions

This patch adds support for the SHA-256 Secure Hash Algorithm for CPUs
that have support for the SHA-256 part of the ARM v8 Crypto Extensions.

It greatly improves sha-256 based operations, about 17x faster on iMX8M
evk board. ~12ms vs ~208ms for a 20MiB kernel sha-256 verification.

asm implementation is a simplified version of the Linux version (from
Ard Biesheuvel).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>