docs/system: ppc: Update the URL for OpenPOWER firmware images

This also fixes a small skiboot/skiroot typo and removes the links to
the specific POWER8 and POWER9 images since the firmware images can be
used to run all machines.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20210902130928.528803-2-clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
1 file changed