dt-bindings: serial: imx: Document mandatory clock properties

The UART IP must be connected to clock, document the properties in DT bindings.
Update example to match Linux arch/arm/boot/dts/imx51.dtsi .
Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230219142250.10176-2-marex@denx.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -49,6 +49,14 @@ properties:
   reg:
     maxItems: 1
 
+  clocks:
+    maxItems: 2
+
+  clock-names:
+    items:
+      - const: ipg
+      - const: per
+
   dmas:
     items:
       - description: DMA controller phandle and request line for RX
@@ -96,12 +104,16 @@ properties:
 required:
   - compatible
   - reg
+  - clocks
+  - clock-names
   - interrupts
 
 unevaluatedProperties: false
 
 examples:
   - |
+    #include <dt-bindings/clock/imx5-clock.h>
+
     aliases {
         serial0 = &uart1;
     };
@@ -110,6 +122,9 @@ examples:
         compatible = "fsl,imx51-uart", "fsl,imx21-uart";
         reg = <0x73fbc000 0x4000>;
         interrupts = <31>;
+        clocks = <&clks IMX5_CLK_UART1_IPG_GATE>,
+                 <&clks IMX5_CLK_UART1_PER_GATE>;
+        clock-names = "ipg", "per";
         dmas = <&sdma 18 4 1>, <&sdma 19 4 2>;
         dma-names = "rx", "tx";
         uart-has-rtscts;