crypto: Moving threading_alt.h to the CC3XX platform

The CC3XX platform contains the functions for mbedtls mutex
handling:
- mbedtls_mutex_init
- mbedtls_mutex_free
- mbedtls_mutex_lock
- mbedtls_mutex_unlock

These functions are not only used by the legacy mbedTLS APIs.
They are also used by the PSA APIs for the PSA core mutexes:
- mbedtls_threading_key_slot_mutex
- mbedtls_threading_psa_globaldata_mutex
- mbedtls_threading_psa_rngdata_mutex

This means that even when the CC3XX legacy/PSA driver is not
used, as long as the CC3XX platform is enabled the mutex
handling will be provided by the CC3XX platform library.
Since the CC3XX platform library provides the implementation
and the threading_alt.h provides the definitions for the mutexes
it makes sense to be in one place.

This commit moves the threading_alt.h to the platform folder
so that it can be included by applications which don't use
the CC3XX mbedcrypto driver but they use Oberon PSA driver.

Ref: NCSDK-31155

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>

Author: Vge0rge