dt-bindings: net: bluetooth: add clock property to UART-based device

Some board requires explicitily control external osscilator via GPIO.
So, add a clock property for an external oscillator for the device.
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

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

@@ -73,6 +73,10 @@ Optional properties:
 - reset-gpios:	GPIO used to reset the device whose initial state keeps low,
 		if the GPIO is missing, then board-level design should be
 		guaranteed.
+- clocks:	Should be the clock specifiers corresponding to the entry in
+		clock-names property. If the clock is missing, then board-level
+		design should be guaranteed.
+- clock-names:	Should contain "osc" entry for the external oscillator.
 - current-speed:  Current baud rate of the device whose defaults to 921600
 
 Example: