ARM: dts: rockchip: add operating points and armclk references

Add basic OPP entries for current supported Rockchip SoCs.
The operating points are currently very conservative, so individual
boards may opt to redefine them.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>