spi: dt-bindings: Add num-cs property for mpfs-spi

The PolarFire SoC SPI "hard" controller supports eight CS lines, out of
which only one CS line is physically wired. The default value of
'num-cs' was never set and it did not didn't impose a maximum value.

To reflect this hardware limitation in the device tree, the binding
enforces that the 'num-cs' property cannot exceed 1 unless additional
CS lines are explicitly defined using GPIO descriptors.

Fixes: 2da187304e ("spi: add bindings for microchip mpfs spi")
Signed-off-by: Prajna Rajendra Kumar <prajna.rajendrakumar@microchip.com>
Link: https://msgid.link/r/20240514104508.938448-2-prajna.rajendrakumar@microchip.com
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/microchip,mpfs-spi.yaml

@@ -13,9 +13,6 @@ description:
 maintainers:
   - Conor Dooley <conor.dooley@microchip.com>
 
-allOf:
-  - $ref: spi-controller.yaml#
-
 properties:
   compatible:
     oneOf:
@@ -43,6 +40,32 @@ required:
   - interrupts
   - clocks
 
+allOf:
+  - $ref: spi-controller.yaml#
+
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: microchip,mpfs-spi
+    then:
+      properties:
+        num-cs:
+          default: 1
+
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: microchip,mpfs-spi
+      not:
+        required:
+          - cs-gpios
+    then:
+      properties:
+        num-cs:
+          maximum: 1
+
 unevaluatedProperties: false
 
 examples: