dt-bindings: pinctrl: rt2880: add proper function muxing binding

Not every function can be muxed to a group. Add proper binding which
documents which function can be muxed to a group or set of groups.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20221231160849.40544-5-arinc.unal@arinc9.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/ralink,rt2880-pinctrl.yaml

@@ -29,18 +29,93 @@ patternProperties:
         $ref: pinmux-node.yaml#
 
         properties:
-          groups:
-            description: The pin group to select.
-            enum: [i2c, spi, uartlite, jtag, mdio, sdram, pci]
-
           function:
-            description: The mux function to select.
+            description:
+              A string containing the name of the function to mux to the group.
             enum: [gpio, i2c, spi, uartlite, jtag, mdio, sdram, pci]
 
+          groups:
+            description:
+              An array of strings. Each string contains the name of a group.
+            maxItems: 1
+
         required:
           - groups
           - function
 
+        allOf:
+          - if:
+              properties:
+                function:
+                  const: gpio
+            then:
+              properties:
+                groups:
+                  enum: [i2c, spi, uartlite, jtag, mdio, sdram, pci]
+
+          - if:
+              properties:
+                function:
+                  const: i2c
+            then:
+              properties:
+                groups:
+                  enum: [i2c]
+
+          - if:
+              properties:
+                function:
+                  const: spi
+            then:
+              properties:
+                groups:
+                  enum: [spi]
+
+          - if:
+              properties:
+                function:
+                  const: uartlite
+            then:
+              properties:
+                groups:
+                  enum: [uartlite]
+
+          - if:
+              properties:
+                function:
+                  const: jtag
+            then:
+              properties:
+                groups:
+                  enum: [jtag]
+
+          - if:
+              properties:
+                function:
+                  const: mdio
+            then:
+              properties:
+                groups:
+                  enum: [mdio]
+
+          - if:
+              properties:
+                function:
+                  const: sdram
+            then:
+              properties:
+                groups:
+                  enum: [sdram]
+
+          - if:
+              properties:
+                function:
+                  const: pci
+            then:
+              properties:
+                groups:
+                  enum: [pci]
+
         additionalProperties: false
 
     additionalProperties: false
@@ -54,7 +129,6 @@ required:
 additionalProperties: false
 
 examples:
-  # Pinmux controller node
   - |
     pinctrl {
       compatible = "ralink,rt2880-pinctrl";