ppc/pnv: Add a comment on the "primary-topology-index" property

ppc/pnv: Add a comment on the "primary-topology-index" property

On P10, the chip id is calculated from the "Primary topology table
index". See skiboot commits for more information [1].

This information is extracted from the hdata on real systems which
QEMU needs to emulate. Add this property for all machines even if it
is only used on POWER10.

[1] https://github.com/open-power/skiboot/commit/2ce3f083f399
    https://github.com/open-power/skiboot/commit/a2d4d7f9e14a

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20210901094153.227671-4-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>