arm64: dts: rockchip: rk3399: Rename OPP nodes as opp@<opp-hz>

arm64: dts: rockchip: rk3399: Rename OPP nodes as opp@<opp-hz>

It would be better to name OPP nodes as opp@<opp-hz> as that will ensure
that multiple DT nodes don't contain the same frequency. Of course we
expect the writer to name the node with its opp-hz frequency and not any
other frequency.

And that will let the compile error out if multiple nodes are using the
same opp-hz frequency.

Change-Id: I8c77646329e39390fb135d4d75d34893a8168876
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>