ARM: dts: rockchip: fix rk3288 cpu opp node reference

The following error can be seen during boot:

  of: /cpus/cpu@501: Couldn't find opp node

Change cpu nodes to use operating-points-v2 in order to fix this.

Fixes: ce76de98 ("ARM: dts: rockchip: convert rk3288 to operating-points-v2")
Cc: stable@vger.kernel.org
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>