tcgbios: Use assembly for 32 bit rotr in sha256

Use assembly for the 32 bit rotr in the sha256 implementation
similar to the assembly used in the sha1 and sha512 implementations.

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