modules: openthread: use PSA API by default

Make CONFIG_OPENTHREAD_CRYPTO_PSA as enabled by default in order to use
PSA Crypto API instead of legacy Mbed TLS' crypto. This is done with the
final goal to deprecate and remove usage of legacy crypto support in
the near future from Zephyr codebase.

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