crypto: arm/ghash - Make the "ghash" crypto_shash NEON-only

crypto: arm/ghash - Make the "ghash" crypto_shash NEON-only

arch/arm/crypto/ghash-ce-glue.c originally provided only a "ghash"
crypto_shash algorithm using PMULL if available, else NEON.

Significantly later, it was updated to also provide a full AES-GCM
implementation using PMULL.

This made the PMULL support in the "ghash" crypto_shash largely
obsolete.  Indeed, the arm64 equivalent of this file unconditionally
uses only ASIMD in its "ghash" crypto_shash.

Given that inconsistency and the fact that the NEON-only code is more
easily separable into the GHASH library than the PMULL based code is,
let's align with arm64 and just support NEON-only for the pure GHASH.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260319061723.1140720-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>