Added support for the CHACHA20 skcipher algorithm.
Tested on an eip197c-iesb configuration in the Xilinx VCU118 devboard,
passes all testmgr vectors plus the extra fuzzing tests.

changes since v1:
- rebased on top of DES library changes done on cryptodev/master
- fixed crypto/Kconfig so that generic fallback is compiled as well

changes since v2:
- made switch entry SAFEXCEL_AES explit and added empty default, as
  requested by Antoine Tenart. Also needed to make SM4 patches apply.

changes since v3:
- nothing
Signed-off-by: Pascal van Leeuwen <pvanleeuwen@verimatrix.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>