dt-bindings: net: bluetooth: Add MediaTek MT7921S SDIO Bluetooth

The MediaTek MT7921S is a WiFi/Bluetooth combo chip that works over
SDIO. WiFi and Bluetooth are separate SDIO functions within the chip.
While the Bluetooth SDIO function is fully discoverable, the chip has
a pin that can reset just the Bluetooth core, as opposed to the full
chip. This should be described in the device tree.

Add a device tree binding for the Bluetooth SDIO function of the MT7921S
specifically to document the reset line. This binding is based on the MMC
controller binding, which specifies one device node per SDIO function.

Cc: Sean Wang <[e-mail]>
Signed-off-by: Chen-Yu Tsai <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

Documentation/devicetree/bindings/net/bluetooth/mediatek,mt7921s-bluetooth.yaml

···
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/bluetooth/mediatek,mt7921s-bluetooth.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MediaTek MT7921S Bluetooth
+
+maintainers:
+  - Sean Wang <[e-mail]>
+
+description:
+  MT7921S is an SDIO-attached dual-radio WiFi+Bluetooth Combo chip; each
+  function is its own SDIO function on a shared SDIO interface. The chip
+  has two dedicated reset lines, one for each function core.
+  This binding only covers the Bluetooth SDIO function, with one device
+  node describing only this SDIO function.
+
+allOf:
+  - $ref: bluetooth-controller.yaml#
+
+properties:
+  compatible:
+    enum:
+      - mediatek,mt7921s-bluetooth
+
+  reg:
+    const: 2
+
+  reset-gpios:
+    maxItems: 1
+    description:
+      An active-low reset line for the Bluetooth core; on typical M.2
+      key E modules this is the W_DISABLE2# pin.
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    mmc {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        bluetooth@2 {
+            compatible = "mediatek,mt7921s-bluetooth";
+            reg = <2>;
+            reset-gpios = <&pio 8 GPIO_ACTIVE_LOW>;
+        };
+    };

MAINTAINERS

···
 L:	[e-mail]
 L:	[e-mail] (moderated for non-subscribers)
 S:	Maintained
+F:	Documentation/devicetree/bindings/net/bluetooth/mediatek,mt7921s-bluetooth.yaml
 F:	Documentation/devicetree/bindings/net/mediatek-bluetooth.txt
 F:	drivers/bluetooth/btmtkuart.c