crypto: des - split off DES library from generic DES cipher driver

Another one for the cipher museum: split off DES core processing into
a separate module so other drivers (mostly for crypto accelerators)
can reuse the code without pulling in the generic DES cipher itself.
This will also permit the cipher interface to be made private to the
crypto API itself once we move the only user in the kernel (CIFS) to
this library interface.

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