psa_tls12_prf_key_derivation_state_t: add optional step to set other key
psa_tls12_prf_key_derivation_state_t: add optional step to set other key

Signed-off-by: Przemek Stekiel <1d381786fa074b2f2333843496828c8e4357e75f@mobica.com>

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