wireguard: Kconfig: select CRYPTO_BLAKE2S_ARM

When available, select the new implementation of BLAKE2s for 32-bit ARM.
This is faster than the generic C implementation.

Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>