ppc/pnv: move phbbar to PnvPHB4

ppc/pnv: move phbbar to PnvPHB4

This MemoryRegion is simple enough to be moved in a single step.

A 'stack->phb' pointer had to be introduced in pnv_pec_stk_update_map()
because this function isn't ready to be fully converted to use a PnvPHB4
pointer instead. This will be dealt with in the following patches.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220113192952.911188-4-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>