ppc/pnv: add PHB4 helpers for user created pnv-phb

ppc/pnv: add PHB4 helpers for user created pnv-phb

The PHB4 backend relies on a link with the corresponding PEC element.
This is trivial to do during machine_init() time for default devices,
but not so much for user created ones.

pnv_phb4_get_pec() is a small variation of the function that was
reverted by commit 9c10d86fee "ppc/pnv: Remove user-created PHB{3,4,5}
devices". We'll use it to determine the appropriate PEC for a given user
created pnv-phb that uses a PHB4 backend.

This is done during realize() time, in pnv_phb_user_device_init().

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220811163950.578927-8-danielhb413@gmail.com>