Add a simple helper to convert from PK sigalg to PSA algorithm. This is
handy when calling mbedtls_pk_can_do_psa() knowing the PK sigalg and the
used MD type.
This is being added in a separate file because it's meant to be consumed
by both ssl and x509 modules. It was not added to tf-psa-crypto because
this is only needed on the mbedtls repo and doing so reduce interdependencies
between the repos.
Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
