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>