dt-bindings: timer: Add Loongson-1 clocksource

Add devicetree binding document for Loongson-1 clocksource.
Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230512103724.587760-3-keguang.zhang@gmail.com

Documentation/devicetree/bindings/timer/loongson,ls1x-pwmtimer.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/timer/loongson,ls1x-pwmtimer.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Loongson-1 PWM timer
+
+maintainers:
+  - Keguang Zhang <keguang.zhang@gmail.com>
+
+description:
+  Loongson-1 PWM timer can be used for system clock source
+  and clock event timers.
+
+properties:
+  compatible:
+    const: loongson,ls1b-pwmtimer
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/loongson,ls1x-clk.h>
+    #include <dt-bindings/interrupt-controller/irq.h>
+    clocksource: timer@1fe5c030 {
+        compatible = "loongson,ls1b-pwmtimer";
+        reg = <0x1fe5c030 0x10>;
+
+        clocks = <&clkc LS1X_CLKID_APB>;
+        interrupt-parent = <&intc0>;
+        interrupts = <20 IRQ_TYPE_LEVEL_HIGH>;
+    };