dt-bindings: serial: convert st,asc to DT schema

'clocks' property is required regarding the device. Convert st,asc
binding to DT schema format in order to add this property, and update
example.
Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240226152135.8671-1-rgallaispou@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,asc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/serial/st,asc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: STMicroelectronics STi SoCs Serial Port
+
+maintainers:
+  - Patrice Chotard <patrice.chotard@foss.st.com>
+
+allOf:
+  - $ref: serial.yaml#
+
+properties:
+  compatible:
+    const: st,asc
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  st,hw-flow-ctrl:
+    description: When set, enable hardware flow control.
+    type: boolean
+
+  st,force-m1:
+    description: When set, force asc to be in Mode-1. This is recommended for
+      high bit rates above 19.2K.
+    type: boolean
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/stih407-clks.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    serial@9830000 {
+        compatible = "st,asc";
+        reg = <0x9830000 0x2c>;
+        interrupts = <GIC_SPI 122 IRQ_TYPE_LEVEL_HIGH>;
+        clocks = <&clk_s_c0_flexgen CLK_EXT2F_A9>;
+    };
+...

Documentation/devicetree/bindings/serial/st-asc.txt

-*st-asc(Serial Port)
-
-Required properties:
-- compatible : Should be "st,asc".
-- reg, reg-names, interrupts, interrupt-names	: Standard way to define device
-			resources with names. look in
-			Documentation/devicetree/bindings/resource-names.txt
-
-Optional properties:
-- st,hw-flow-ctrl	bool flag to enable hardware flow control.
-- st,force-m1		bool flat to force asc to be in Mode-1 recommended
-			for high bit rates (above 19.2K)
-Example:
-serial@fe440000{
-    compatible    = "st,asc";
-    reg         = <0xfe440000 0x2c>;
-    interrupts     =  <0 209 0>;
-};