target/ppc: Add Power9/10 power management SPRs

target/ppc: Add Power9/10 power management SPRs

Linux power management code accesses these registers for pstate
management. Wire up a very simple implementation.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
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After OCC fixes in QEMU pnv model and skiboot (since they have suffered
some bitrot), Linux will start performing PM SPR accesses. This is a
very simple implementation that makes it a bit happier.

Thanks,
Nick