hw/ssi: Introduce a ssi_get_cs() helper

hw/ssi: Introduce a ssi_get_cs() helper

Simple routine to retrieve a DeviceState object on a SPI bus using its
CS index. It will be useful for the board to wire the CS lines.

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>