ARM: dts: rockchip: add soc-specific uart compatibles for rk3066/rk3188

The serial IPs in Rockchip socs are based on Designware uarts and thus
bind against the snps,dw-apb-uart compatible.
On all newer socs we also carry around per-soc compatibles that allow
us to have more specific drivers in the future - if needed.

The cortex-a9 socs rk3066 and rk3188 that were added first don't have
those yet, so add them for completenes sake.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>