crypto: ecc - Add nbits field to ecc_curve structure

Add the number of bits a curve has to the ecc_curve definition to be able
to derive the number of bytes a curve requires for its coordinates from it.
It also allows one to identify a curve by its particular size. Set the
number of bits on all curve definitions.

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