dt-bindings: timer: Add bindings for the RISC-V timer device

We add DT bindings for a separate RISC-V timer DT node which can
be used to describe implementation specific behaviour (such as
timer interrupt not triggered during non-retentive suspend).

Signed-off-by: Anup Patel <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Acked-by: Palmer Dabbelt <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Daniel Lezcano <[e-mail]>

Documentation/devicetree/bindings/timer/riscv,timer.yaml

···
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/timer/riscv,timer.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: RISC-V timer
+
+maintainers:
+  - Anup Patel <[e-mail]>
+
+description: |+
+  RISC-V platforms always have a RISC-V timer device for the supervisor-mode
+  based on the time CSR defined by the RISC-V privileged specification. The
+  timer interrupts of this device are configured using the RISC-V SBI Time
+  extension or the RISC-V Sstc extension.
+
+  The clock frequency of RISC-V timer device is specified via the
+  "timebase-frequency" DT property of "/cpus" DT node which is described
+  in Documentation/devicetree/bindings/riscv/cpus.yaml
+
+properties:
+  compatible:
+    enum:
+      - riscv,timer
+
+  interrupts-extended:
+    minItems: 1
+    maxItems: 4096   # Should be enough?
+
+  riscv,timer-cannot-wake-cpu:
+    type: boolean
+    description:
+      If present, the timer interrupt cannot wake up the CPU from one or
+      more suspend/idle states.
+
+additionalProperties: false
+
+required:
+  - compatible
+  - interrupts-extended
+
+examples:
+  - |
+    timer {
+      compatible = "riscv,timer";
+      interrupts-extended = <&cpu1intc 5>,
+                            <&cpu2intc 5>,
+                            <&cpu3intc 5>,
+                            <&cpu4intc 5>;
+    };
+...