PM / OPP: Use - instead of @ for DT entries

Compiling the DT file with W=1, DTC warns like follows:

Warning (unit_address_vs_reg): Node /opp_table0/opp@1000000000 has a
unit name, but no reg property

Fix this by replacing '@' with '-' as the OPP nodes will never have a
"reg" property.

Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>