crypto: aesni - add generic gcm(aes)

Now that the asm side of things can support all the valid lengths of ICV
and all lengths of associated data, provide the glue code to expose a
generic gcm(aes) crypto algorithm.
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>