Expose higher-level interface to PM registers via PlatformInfo

[IsaacWoods]

It would be good to expose a higher-level API to the PM register blocks, probably via PlatformInfo as that already reads the FADT. This would ideally group the PM1 event and control, the PM2, and perhaps the PM timer register blocks (although the timer would be a breaking change, so maybe keep as is).

cc @semiviral