dt-bindings: serial: 8250: Add MediaTek BTIF controller bindings

Document the devicetree bindings in 8250.txt for MediaTek BTIF
controller which could be found on MT7622 and MT7623 SoC.
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/8250.txt

@@ -14,6 +14,8 @@ Required properties:
 	  tegra132, or tegra210.
 	- "nxp,lpc3220-uart"
 	- "ralink,rt2880-uart"
+	- For MediaTek BTIF, must contain '"mediatek,<chip>-btif",
+	  "mediatek,mtk-btif"' where <chip> is mt7622, mt7623.
 	- "altr,16550-FIFO32"
 	- "altr,16550-FIFO64"
 	- "altr,16550-FIFO128"