crypto: arm/ghash - use variably sized key struct

Of the two versions of GHASH that the ARM driver implements, only one
performs aggregation, and so the other one has no use for the powers
of H to be precomputed, or space to be allocated for them in the key
struct. So make the context size dependent on which version is being
selected, and while at it, use a static key to carry this decision,
and get rid of the function pointer.

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