crypto: sm4 - export encrypt/decrypt routines to other drivers

In preparation of adding support for the SIMD based arm64 implementation
of arm64, which requires a fallback to non-SIMD code when invoked in
certain contexts, expose the generic SM4 encrypt and decrypt routines
to other drivers.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>