ppc/pnv: Implement POWER10 PC xscom registers for direct controls

ppc/pnv: Implement POWER10 PC xscom registers for direct controls

The PC unit in the processor core contains xscom registers that provide
low level status and control of the CPU.

This implements "direct controls", sufficient for skiboot firmware,
which uses it to send NMI IPIs between CPUs.

POWER10 is sufficiently different from POWER9 (particularly with respect
to QME and special wakeup) that it is not trivial to implement POWER9
support by reusing the code.

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