ppc/pnv: Add a big-core mode that joins two regular cores

ppc/pnv: Add a big-core mode that joins two regular cores

POWER9 and POWER10 machines come in two variants, big-core and
small-core. Big-core machines are SMT8 from software's point of view,
but the low level platform topology ("xscom registers and pervasive
addressing"), these look more like a pair of small cores ganged
together.

Presently the way this is modelled is to create one SMT8 PnvCore and add
special cases to xscom and pervasive for big-core mode that tries to
split this into two small cores, but this is becoming too complicated to
manage.

A better approach is to create 2 core structures and ganging them
together to look like an SMT8 core in TCG. Then the xscom and pervasive
models mostly do not need to differentiate big and small core modes.

This change adds initial mode bits and QEMU topology handling to
split SMT8 cores into 2xSMT4 cores.

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