staging: mt7621-pci: dt-bindings: add perst-gpio to sample bindings

Both driver and device tree have been updated to use GPIO perst.
Update bindings documentation also.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-pci/mediatek,mt7621-pci.txt

@@ -6,6 +6,7 @@ Required properties:
 - reg: Base addresses and lengths of the PCIe subsys and root ports.
 - bus-range: Range of bus numbers associated with this controller.
 - #address-cells: Address representation for root ports (must be 3)
+- perst-gpio: PCIe reset signal line.
 - pinctrl-names : The pin control state names.
 - pinctrl-0: The "default" pinctrl state.
 - #size-cells: Size representation for root ports (must be 2)
@@ -48,6 +49,7 @@ Example for MT7621:
 		#address-cells = <3>;
 		#size-cells = <2>;
 
+		perst-gpio = <&gpio 19 GPIO_ACTIVE_HIGH>;
 		pinctrl-names = "default";
 		pinctrl-0 = <&pcie_pins>;