crypto: arm64/gcm - use variably sized key struct

Now that the ghash and gcm drivers are split, we no longer need to allocate
a key struct for the former that carries powers of H that are only used by
the latter. Also, take this opportunity to clean up the code a little bit.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>