powerpc/powernv/ioda: Find opencapi slot for a device node

powerpc/powernv/ioda: Find opencapi slot for a device node

BugLink: https://bugs.launchpad.net/bugs/1862121
Unlike real PCI slots, opencapi slots are directly associated to
the (virtual) opencapi PHB, there's no intermediate bridge. So when
looking for a slot ID, we must start the search from the device node
itself and not its parent.

Also, the slot ID is not attached to a specific bdfn, so let's build
it from the PHB ID, like skiboot.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191121134918.7155-6-fbarrat@linux.ibm.com
(cherry picked from commit bbb789046084d18fff1a32a7bcdeca36dfa0ac14)
Signed-off-by: Frank Heimes <frank.heimes@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>