staging: wfx: introduce "secure link"

staging: wfx: introduce "secure link"

Chip support encryption of the link between host and chip. This feature
is called "secure link". Driver code on github[1] support it. However,
it relies on mbedtls for cryptographic functions. So, I decided to not
import this feature in current patch. However, in order to keep code
synchronized between github and kernel, I imported all code related to
this feature, even if most of it is just no-op.

[1]: https://github.com/SiliconLabs/wfx-linux-driver/

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20190919142527.31797-14-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>