crypto: crypto4xx - move and refactor dynamic_contents helpers

This patch refactors and moves the dynamic_contents helper
functions into the crypto4xx_sa.h header file.

 * get_dynamic_sa_iv_size is no longer needed, as the cryptoapi
   provides the required IV size information as well.

 * refactor the function declarations to use the a pointer to the
   dynamic_sa_contents union, instead of the crypto4xx_ctx.

 * rename get_dynamic_sa_offset_key_field to get_dynamic_sa_key_field.
   It returns the pointer to the key directly.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>