ppc/pnv: move nest_regs_mr to PnvPHB4

ppc/pnv: move nest_regs_mr to PnvPHB4

We're now able to cleanly move nest_regs_mr to the PnvPHB4 device.

One thing of notice here is the need to use a phb->stack->pec pointer
because pnv_pec_stk_nest_xscom_write requires a PEC object. Another
thing that can be noticed in the use of 'stack->stack_no' that still
remains throughout the XSCOM code.

After moving all MemoryRegions to the PnvPHB4 object, this illustrates
what is the remaining role of the stack: provide a PEC pointer and the
'stack_no' information. If we can provide these in the PnvPHB4 object
instead (spoiler: we can, and we will), the PnvPhb4PecStack device will
be deprecated and can be removed.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220113192952.911188-10-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>