Add usage info of generate_psa_constants script
Add usage info of generate_psa_constants script

Signed-off-by: Hugues de Valon <a9b22d551ecd4fafac6f424ca9eafc4fb132330a@arm.com>

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