Add function to retrieve the tls_prf type
Add function to retrieve the tls_prf type

Add `tls_prf_get_type()` static function that returns the
`mbedtls_tls_prf_types` according to the used `tls_prf` function.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls