Define standard arm UUIDs

[samerhaj]

Need to add the following standard Arm _DSD UUIDs to the spec:

• SCMI Device Properties UUID (https://developer.arm.com/documentation/111115/latest): "84a2d1c6-86b6-4199-8dac-9c17449d5e03"
• FFA Device Properties UUID (https://developer.arm.com/documentation/den0077/latest): "c08c3233-b316-4723-a9d7-e21b7ac0fb6a"

This replaces the previous proposal in Issue #29