crypto: x86/chacha - depend on generic chacha library instead of crypto driver

In preparation of extending the x86 ChaCha driver to also expose the ChaCha
library interface, drop the dependency on the chacha_generic crypto driver
as a non-SIMD fallback, and depend on the generic ChaCha library directly.
This way, we only pull in the code we actually need, without registering
a set of ChaCha skciphers that we will never use.

Since turning the FPU on and off is cheap these days, simplify the SIMD
routine by dropping the per-page yield, which makes for a cleaner switch
to the library API as well. This also allows use to invoke the skcipher
walk routines in non-atomic mode.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>