crypto: ecdsa - Register NIST P521 and extend test suite

Register NIST P521 as an akcipher and extend the testmgr with
NIST P521-specific test vectors.

Add a module alias so the module gets automatically loaded by the crypto
subsystem when the curve is needed.

Tested-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>