crypto: aegis128 - expose SIMD code path as separate driver

Wiring the SIMD code into the generic driver has the unfortunate side
effect that the tcrypt testing code cannot distinguish them, and will
therefore not use the latter to fuzz test the former, as it does for
other algorithms.

So let's refactor the code a bit so we can register two implementations:
aegis128-generic and aegis128-simd.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Ondrej Mosnacek <omosnacek@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>