crypto: arm/ghash-ce - add missing async import/export

Since commit 8996eafd ("crypto: ahash - ensure statesize is non-zero"),
all ahash drivers are required to implement import()/export(), and must have
a non-zero statesize. Fix this for the ARM Crypto Extensions GHASH
implementation.

Fixes: 8996eafd ("crypto: ahash - ensure statesize is non-zero")
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>