crypto: powerpc/poly1305 - implement library instead of shash

Currently the Power10 optimized Poly1305 is only wired up to the
crypto_shash API, which makes it unavailable to users of the library
API.  The crypto_shash API for Poly1305 is going to change to be
implemented on top of the library API, so the library API needs to be
supported.  And of course it's needed anyway to serve the library users.

Therefore, change the Power10 optimized Poly1305 code to implement the
library API instead of the crypto_shash API.

Cc: Danny Tsen <dtsen@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>