cpufreq: dt: Mark platdev machines array as __initconst

The machines array in cpufreq-dt-platdev is used only once at boot time
and so should be marked with __initconst, so that kernel can free up
memory used for it, if required.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>