crypto: chacha20poly1305 - export CHACHAPOLY_IV_SIZE

Move CHACHAPOLY_IV_SIZE to header file, so it can be reused.

Signed-off-by: Cristian Stoica <cristian.stoica@nxp.com>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>