arm64: dts: rockchip: add max-link-speed for rk3399

Per the errata of TRM, rk3399 won't support gen2 from
now on, so let's set max-link-speed to 1 in order not
to doing training for gen2.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399.dtsi

@@ -297,6 +297,7 @@ pcie0: pcie@f8000000 {
 				<0 0 0 2 &pcie0_intc 1>,
 				<0 0 0 3 &pcie0_intc 2>,
 				<0 0 0 4 &pcie0_intc 3>;
+		max-link-speed = <1>;
 		msi-map = <0x0 &its 0x0 0x1000>;
 		phys = <&pcie_phy>;
 		phy-names = "pcie-phy";