ARM: dts: rockchip: add serial aliases for rk3066 and rk3188

Add aliases for UARTs on rk3066 and rk3188 in order to fix the numbering scheme.
This will keep the debug console on ttyS2 when UART 1 is disabled, for example.

Signed-off-by: Julien CHAUVEAU <julien.chauveau@neo-technologies.fr>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>