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

Not every platform has the pinctrl device integrates the GPIO the function
such as MT7621 whose pinctrl and GPIO are separate hardware so adding an
additional boot-gpios property for such platform allows them to bring up
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

@@ -50,11 +50,24 @@ Required properties:
 		  "mediatek,mt7663u-bluetooth": for MT7663U device
 		  "mediatek,mt7668u-bluetooth": for MT7668U device
 - vcc-supply:	Main voltage regulator
+
+If the pin controller on the platform can support both pinmux and GPIO
+control such as the most of MediaTek platform. Please use below properties.
+
 - pinctrl-names: Should be "default", "runtime"
 - pinctrl-0: Should contain UART RXD low when the device is powered up to
 	     enter proper bootstrap mode.
 - pinctrl-1: Should contain UART mode pin ctrl
 
+Else, the pin controller on the platform only can support pinmux control and
+the GPIO control still has to rely on the dedicated GPIO controller such as
+a legacy MediaTek SoC, MT7621. Please use the below properties.
+
+- boot-gpios:	GPIO same to the pin as UART RXD and used to keep LOW when
+		the device is powered up to enter proper bootstrap mode when
+- pinctrl-names: Should be "default"
+- pinctrl-0: Should contain UART mode pin ctrl
+
 Optional properties:
 
 - reset-gpios:	GPIO used to reset the device whose initial state keeps low,