SPCI: Misc protocol definitions.
SPCI: Misc protocol definitions.

- message header
- SPCI protocol ids
- SPCI return codes
- helper methods

Change-Id: I2681b4025f66471098937527d254c21cdd761639

project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification