hw/ssi: Add a "cs" property to SSIPeripheral

Boards will use this new property to identify the device CS line and
wire the SPI controllers accordingly.

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>

include/hw/ssi/ssi.h

@@ -64,6 +64,9 @@ struct SSIPeripheral {
 
     /* Chip select state */
     bool cs;
+
+    /* Chip select index */
+    uint8_t cs_index;
 };
 
 extern const VMStateDescription vmstate_ssi_peripheral;