net: tls_credentials: use PSA to compute SHA256 when BUILD_WITH_TFM

When using the TLS_CREDENTIALS_BACKEND_PROTECTED_STORAGE there is
BUILD_WITH_TFM enabled which means that we can take advantage of
TFM (thorugh PSA interface) to compute the SHA256 hash.

This removes the need for the built-in implementation of
mbedtls_sha256() on the non-secure image (unless it's used somewhere
else, of course) and therefore it helps in reducing the memory
footprint.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>