crypto: tcrypt - add ffdhe2048(dh) test

Commit 7dce5981 ("crypto: dh - implement ffdheXYZ(dh) templates")
implemented the said templates. Add ffdhe2048(dh) test as it is the
fastest one. This is a requirement for the FIPS certification.

Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>