ARM: dts: exynos4412: Rename OPP nodes as opp@<opp-hz>

OPP bindings got updated to name OPP nodes this way, make changes
according to that.

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>