Add HMAC-SHA256 support in CESA

Only HMAC-SHA256 is added as it is the only SHA-2 variant supported by
cryptodev. It is not possible to register hardware support for other
algorithms in the family including regular non-keyed SHA256.

Submitted by: Michal Stanek <mst@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D6219