bsp: Add protobuf interface to se-proxy-sp

Adds protobuf interface to se-proxy-sp as the main crypto-sp uses it and
parsec service 1.4 also switch using protobuf interface.

Signed-off-by: Emekcan Aras <emekcan.aras@arm.com>
Signed-off-by: Adam Johnston <adam.johnston@arm.com>