arm64: dts: rockchip: default serial for Firefly-RK3399

The Firefly-RK3399 uses serial2 with 1,500,000 baud by default
for communication in U-Boot and in the vendor provided distros.

So let us set the same default in the Linux kernel.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>