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: NHeiko Stuebner <heiko@sntech.de>