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>

hw/ppc/pnv_xscom.c

@@ -284,6 +284,10 @@ int pnv_dt_xscom(PnvChip *chip, void *fdt, int root_offset,
     _FDT(xscom_offset);
     g_free(name);
     _FDT((fdt_setprop_cell(fdt, xscom_offset, "ibm,chip-id", chip->chip_id)));
+    /*
+     * On P10, the xscom bus id has been deprecated and the chip id is
+     * calculated from the "Primary topology table index". See skiboot.
+     */
     _FDT((fdt_setprop_cell(fdt, xscom_offset, "ibm,primary-topology-index",
                            chip->chip_id)));
     _FDT((fdt_setprop_cell(fdt, xscom_offset, "#address-cells", 1)));