crypto: arm64/aes-xctr - Improve readability of XCTR and CTR modes

Added some clarifying comments, changed the register allocations to make
the code clearer, and added register aliases.

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>