ARM64: dts: rockchip: Rename OPP nodes as opp@<opp-hz>

ARM64: dts: rockchip: 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: Icefba93f7a95752e344b5a092a83931bf4d1e682
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>