pnv_phb4_pec: Keep track of instantiated PHBs

pnv_phb4_pec: Keep track of instantiated PHBs

Add an array on the PEC object to keep track of the PHBs which are
instantiated. The array can be sparsely populated when using
user-created PHBs. It will be useful for the next patch to only export
instantiated PHBs in the device tree.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20230302163715.129635-2-fbarrat@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>