dt-bindings: serial: imx: Properly describe the i.MX1 interrupts

i.MX1 has three UART interrupts instead of a single one like other
i.MX devices.

Take this into account for properly describing the i.MX1 UART
interrupts.

This fixes the following dt-schema warning:

from schema $id: http://devicetree.org/schemas/serial/fsl-imx-uart.yaml#yaml#

imx1-ads.dtb: serial@206000: interrupts: [[30], [29], [26]] is too long
Signed-off-by: Fabio Estevam <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml

···
 maintainers:
   - Fabio Estevam <[e-mail]>
 
-allOf:
-  - $ref: serial.yaml#
-  - $ref: rs485.yaml#
-
 properties:
   compatible:
     oneOf:
···
       - const: tx
 
   interrupts:
-    maxItems: 1
+    items:
+      - description: UART RX Interrupt
+      - description: UART TX Interrupt
+      - description: UART RTS Interrupt
+    minItems: 1
 
   wakeup-source: true
 
···
   - clocks
   - clock-names
   - interrupts
+
+allOf:
+  - $ref: serial.yaml#
+  - $ref: rs485.yaml#
+
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: fsl,imx1-uart
+    then:
+      properties:
+        interrupts:
+          minItems: 3
+          maxItems: 3
+    else:
+      properties:
+        interrupts:
+          maxItems: 1
 
 unevaluatedProperties: false