crypto: lib/poly1305 - Build main library on LIB_POLY1305 and split generic code out

Split the lib poly1305 code just as was done with sha256.  Make
the main library code conditional on LIB_POLY1305 instead of
LIB_POLY1305_GENERIC.

Reported-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Fixes: 10a6d72e ("crypto: lib/poly1305 - Use block-only interface")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>