ppc/pnv: remove stack pointer from PnvPHB4

This pointer was being used for two reasons: pnv_phb4_update_regions()
was using it to access the PHB and phb4_realize() was using it as a way
to determine if the PHB was user created.

We can determine if the PHB is user created via phb->pec, introduced in
the previous patch, and pnv_phb4_update_regions() is no longer using
stack->phb.

Remove the pointer from the PnvPHB4 device.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220114180719.52117-4-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>