target/ppc: Move SPR indirect registers into PnvCore

target/ppc: Move SPR indirect registers into PnvCore

SPRC/SPRD were recently added to all BookS CPUs supported, but
they are only tested on POWER9 and POWER10, so restrict them to
those CPUs.

SPR indirect scratch registers presently replicated per-CPU like
SMT SPRs, but the PnvCore is a better place for them since they
are restricted to P9/P10.

Also add SPR indirect read access to core thread state for POWER9
since skiboot accesses that when booting to check for big-core
mode.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>