ppc/pnv: change pnv_pec_stk_update_map() to use PnvPHB4

ppc/pnv: change pnv_pec_stk_update_map() to use PnvPHB4

stack->nest_regs_mr wasn't migrated to PnvPHB4 together with phb->nest_regs[] in
the previous patch. We were unable to cleanly convert its write MemoryRegionOps,
pnv_pec_stk_nest_xscom_write(), to use PnvPHB4 instead of PnvPhb4PecStack due to
pnv_pec_stk_update_map() using a stack. Thing is, we're now able to convert
pnv_pec_stk_update_map() because of what the did in previous patch.

The need for this intermediate step is a good example of the interconnected
relationship between stack and phb that we aim to cleanup.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220113192952.911188-9-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>