pnv_phb4_pec: use pnv_phb4_pec_get_phb_id() in pnv_pec_dt_xscom()

Relying on stack->phb to write the xscom DT of the PEC is something that
we won't be able to do with user creatable pnv-phb4 devices.

Hopefully, this can be done by using pnv_phb4_pec_get_phb_id(), which is
already used by pnv_pec_realize() to set the phb-id of the stack. Use
the same idea in pnv_pec_dt_xscom() to write ibm,phb-index without the
need to accessing stack->phb, since stack->phb is not granted to be !=
NULL when user creatable phbs are introduced.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220110143346.455901-4-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>