dt-bindings: mt8173-mtu3: add generic compatible and rename file

The mt8173-mtu3.txt actually holds the bindings for all mediatek
SoCs with usb3 DRD IP, so add a generic compatible and change the
name to mediatek,mtu3.txt.
Signed-off-by: NChunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/mt8173-mtu3.txt → Documentation/devicetree/bindings/usb/mediatek,mtu3.txt

 The device node for Mediatek USB3.0 DRD controller
 
 Required properties:
- - compatible : should be "mediatek,mt8173-mtu3"
+ - compatible : should be "mediatek,<soc-model>-mtu3", "mediatek,mtu3",
+	soc-model is the name of SoC, such as mt8173, mt2712 etc,
+	when using "mediatek,mtu3" compatible string, you need SoC specific
+	ones in addition, one of:
+	- "mediatek,mt8173-mtu3"
  - reg : specifies physical base address and size of the registers
  - reg-names: should be "mac" for device IP and "ippc" for IP port control
  - interrupts : interrupt used by the device IP