ARM: dts: rockchip: Remove 1.8 GHz operation point from phycore som

Rockchip recommends to run the CPU cores only with operations points of
1.6 GHz or lower.

Removed the cpu0 node with too high operation points and use the default
values instead.

Fixes: 903d31e3

 ("ARM: dts: rockchip: Add support for phyCORE-RK3288 SoM")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>