dt-bindings: arm: Convert QEMU fw-cfg to DT schema

Convert the QEMU fw-cfg binding to DT schema format. As this binding is
also used on Risc-V now, drop any architecture references and move to a
common location. The fw-cfg interface has also gained some DMA support
which is coherent, so add the missing 'dma-coherent'.
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Link: https://lore.kernel.org/r/20220310013552.549590-1-robh@kernel.org

Documentation/devicetree/bindings/arm/fw-cfg.txt

-* QEMU Firmware Configuration bindings for ARM
-
-QEMU's arm-softmmu and aarch64-softmmu emulation / virtualization targets
-provide the following Firmware Configuration interface on the "virt" machine
-type:
-
-- A write-only, 16-bit wide selector (or control) register,
-- a read-write, 64-bit wide data register.
-
-QEMU exposes the control and data register to ARM guests as memory mapped
-registers; their location is communicated to the guest's UEFI firmware in the
-DTB that QEMU places at the bottom of the guest's DRAM.
-
-The authoritative guest-side hardware interface documentation to the fw_cfg
-device can be found in "docs/specs/fw_cfg.txt" in the QEMU source tree.
-
-
-Required properties:
-
-- compatible: "qemu,fw-cfg-mmio".
-
-- reg: the MMIO region used by the device.
-  * Bytes 0x0 to 0x7 cover the data register.
-  * Bytes 0x8 to 0x9 cover the selector register.
-  * Further registers may be appended to the region in case of future interface
-    revisions / feature bits.
-
-Example:
-
-/ {
-	#size-cells = <0x2>;
-	#address-cells = <0x2>;
-
-	fw-cfg@9020000 {
-		compatible = "qemu,fw-cfg-mmio";
-		reg = <0x0 0x9020000 0x0 0xa>;
-	};
-};

Documentation/devicetree/bindings/firmware/qemu,fw-cfg-mmio.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/firmware/qemu,fw-cfg-mmio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: QEMU Firmware Configuration bindings
+
+maintainers:
+  - Rob Herring <robh@kernel.org>
+
+description: |
+  Various QEMU emulation / virtualization targets provide the following
+  Firmware Configuration interface on the "virt" machine type:
+
+  - A write-only, 16-bit wide selector (or control) register,
+  - a read-write, 64-bit wide data register.
+
+  QEMU exposes the control and data register to guests as memory mapped
+  registers; their location is communicated to the guest's UEFI firmware in the
+  DTB that QEMU places at the bottom of the guest's DRAM.
+
+  The authoritative guest-side hardware interface documentation to the fw_cfg
+  device can be found in "docs/specs/fw_cfg.txt" in the QEMU source tree.
+
+
+properties:
+  compatible:
+    const: qemu,fw-cfg-mmio
+
+  reg:
+    maxItems: 1
+    description: |
+      * Bytes 0x0 to 0x7 cover the data register.
+      * Bytes 0x8 to 0x9 cover the selector register.
+      * Further registers may be appended to the region in case of future interface
+        revisions / feature bits.
+
+  dma-coherent: true
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+
+    fw-cfg@9020000 {
+        compatible = "qemu,fw-cfg-mmio";
+        reg = <0x9020000 0xa>;
+    };
+...