crypto: arm64/gcm - disentangle ghash and gcm setkey() routines

The remaining ghash implementation does not support aggregation, and so
there is no point in including the precomputed powers of H in the key
struct. So move that into the GCM setkey routine, and get rid of the
shared sub-routine entirely.

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