tpm: Add sha256 implementation

The following patch adds a SHA256 implementation based on the algorithm
description in NIST FIPS PUB 180-4. The patch includes test cases that test
the sha256 implementation and pass on big and little endian ppc64 hosts.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>