ARM: dts: rockchip: default serial for rk3288 Tinker Board

The Asus Tinker Board uses serial 2 with 115,200 baud by default for
communication in U-Boot. The same value is also chosen for other RK3288
boards.

So let us set the same value in the Tinker Board device tree.

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