dt-bindings: net: bluetooth: Add mediatek-bluetooth

Add binding document for a SoC built-in device using MediaTek protocol.
Which could be found on MT7622 SoC or other similar MediaTek SoCs.
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

Documentation/devicetree/bindings/net/mediatek-bluetooth.txt

+MediaTek SoC built-in Bluetooth Devices
+==================================
+
+This device is a serial attached device to BTIF device and thus it must be a
+child node of the serial node with BTIF. The dt-bindings details for BTIF
+device can be known via Documentation/devicetree/bindings/serial/8250.txt.
+
+Required properties:
+
+- compatible:	Must be
+		  "mediatek,mt7622-bluetooth": for MT7622 SoC
+- clocks:	Should be the clock specifiers corresponding to the entry in
+		clock-names property.
+- clock-names:	Should contain "ref" entries.
+- power-domains: Phandle to the power domain that the device is part of
+
+Example:
+
+	btif: serial@1100c000 {
+		compatible = "mediatek,mt7622-btif",
+			     "mediatek,mtk-btif";
+		reg = <0 0x1100c000 0 0x1000>;
+		interrupts = <GIC_SPI 90 IRQ_TYPE_LEVEL_LOW>;
+		clocks = <&pericfg CLK_PERI_BTIF_PD>;
+		clock-names = "main";
+		reg-shift = <2>;
+		reg-io-width = <4>;
+
+		bluetooth {
+			compatible = "mediatek,mt7622-bluetooth";
+			power-domains = <&scpsys MT7622_POWER_DOMAIN_WB>;
+			clocks = <&clk25m>;
+			clock-names = "ref";
+		};
+	};