Add support to use TSS with Zephyr

[cplappert]

WIP to prepare TSS for Zephyr integration.

@drandreas: I would like to have your opinion on this. I noticed that you introduced Zephyr some time ago. This commit overwrites the void* TSS2_TCTI_POLL_HANDLE, since it was not working for me. Would this be a problem from your side?

[drandreas]

Hi @cplappert thank you for reaching out. It has been a while since did those changes. I also changed to a different team so I'm not up-to-date on the Zephyr integration.

I've reached out to a colleague who is still working on the project. I received the following response (they still use the tpm-tss Zephyr integration): "
The patch is fine in my opinion. I don't understand its description in doc/tss-for-zephyr.md and it seems too complicated to me.

TSS2_TCTI_POLL_HANDLE simply has to be defined. It is only used as a parameter for get_poll_handles which only returns TSS2_TCTI_RC_NOT_IMPLEMENTED.

It is somewhat unattractive that UNUSED is now defined twice with the #include <zephyr/posix/poll.h> and the following warning is issued very often:
.../modules/lib/tpm2-tss/import/src/util/aux_util.h:33: warning: "UNUSED" redefined Line 33: #define UNUSED(x) (void)(x)
"