PM / OPP: Update bindings to make opp-hz a 64 bit value

With a 32 bit value, the maximum frequency that the bindings can
support is ~ 4 GHz. And that might fall short of what newer systems may
have.

Allow opp-hz to be a 64 bit big-endian value.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Suggested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>