crypto: arm64/aes - replace scalar fallback with plain NEON fallback

The new bitsliced NEON implementation of AES uses a fallback in two
places: CBC encryption (which is strictly sequential, whereas this
driver can only operate efficiently on 8 blocks at a time), and the
XTS tweak generation, which involves encrypting a single AES block
with a different key schedule.

The plain (i.e., non-bitsliced) NEON code is more suitable as a fallback,
given that it is faster than scalar on low end cores (which is what
the NEON implementations target, since high end cores have dedicated
instructions for AES), and shows similar behavior in terms of D-cache
footprint and sensitivity to cache timing attacks. So switch the fallback
handling to the plain NEON driver.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>