spi: dt-bindings: support devices with multiple chipselects

Add binding support for devices, that have more than one
chip select. A typical example are SPI connected microcontroller,
that can also be programmed over SPI like NXP Kinetis or
chips with a configuration and a data chip select, such as
Microchip's MRF89XA transceiver.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20210621175359.126729-3-sebastian.reichel@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-controller.yaml

@@ -114,8 +114,11 @@ patternProperties:
           Compatible of the SPI device.
 
       reg:
-        minimum: 0
-        maximum: 256
+        minItems: 1
+        maxItems: 256
+        items:
+          minimum: 0
+          maximum: 256
         description:
           Chip select used by the device.