crypto: vmx - Improved AES/XTS performance of 6-way unrolling for ppc

Improve AES/XTS performance of 6-way unrolling for PowerPC up
to 17% with tcrypt.  This is done by using one instruction,
vpermxor, to replace xor and vsldoi.

The same changes were applied to OpenSSL code and a pull request was
submitted.

This patch has been tested with the kernel crypto module tcrypt.ko and
has passed the selftest.  The patch is also tested with
CONFIG_CRYPTO_MANAGER_EXTRA_TESTS enabled.
Signed-off-by: Danny Tsen <dtsen@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>