crypto: arm64/aes-blk - improve XTS mask handling

The Crypto Extension instantiation of the aes-modes.S collection of
skciphers uses only 15 NEON registers for the round key array, whereas
the pure NEON flavor uses 16 NEON registers for the AES S-box.

This means we have a spare register available that we can use to hold
the XTS mask vector, removing the need to reload it at every iteration
of the inner loop.

Since the pure NEON version does not permit this optimization, tweak
the macros so we can factor out this functionality. Also, replace the
literal load with a short sequence to compose the mask vector.

On Cortex-A53, this results in a ~4% speedup.

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