ARM: dts: rockchip: Fix gpu opp node names for rk3288

The device tree compiler yells like this:
  Warning (unit_address_vs_reg):
  /gpu-opp-table/opp@100000000:
  node has a unit name, but no reg property

Let's match the cpu opp node names and use a dash.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>