net: tcp: use PSA for SHA256 when BUILD_WITH_TFM

When BUILD_WITH_TFM is enabled we can dispatch hash computation
to TFM. This allows to remove the built-in support of SHA256 from
the non-secure side (if it's not used for any other purpose, of course).

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