ACPI: arm64: export acpi_arch_thermal_cpufreq_pctg()

The cpufreq code can be in a loadable module, so the architecture support
for it has to be exported:

ERROR: modpost: "acpi_arch_thermal_cpufreq_pctg" [drivers/acpi/processor.ko] undefined!

Fixes: 310293a2

 ("ACPI: processor: reduce CPUFREQ thermal reduction pctg for Tegra241")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>