hw/ssi: Check for duplicate CS indexes

hw/ssi: Check for duplicate CS indexes

This to avoid indexes conflicts on the same SSI bus. Adapt machines
using multiple devices on the same bus to avoid breakage.

Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>