dev-arm: add FVP Base Power Controller model

dev-arm: add FVP Base Power Controller model

This is a reduced model of the FVP Base platforms Power Controller.
As of now it allows the following functions from software:
- Checking for core presence
- Reporting the power state of a core / cluster
- Explicitly powering off a core on WFI
- Explicitly powering off cores in a CPU cluster on WFI
- Explicitly powering on a core through writes to PPONR register

Change-Id: Ia1d4d3ae8e4bfb2d23b2c6077396a4d8500e2e30
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26463
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>