crypto: x86/sha - Restructure x86 sha256 glue code to expose all the available sha256 transforms

Restructure the x86 sha256 glue code so we will expose sha256 transforms
based on SSSE3, AVX, AVX2 or SHA-NI extension as separate individual
drivers when cpu provides such support. This will make it easy for
alternative algorithms to be used if desired and makes the code cleaner
and easier to maintain.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>