cpufreq: CPPC: add ACPI_PROCESSOR dependency

Without the Kconfig dependency, we can get this warning:

warning: ACPI_CPPC_CPUFREQ selects ACPI_CPPC_LIB which has unmet direct dependencies (ACPI && ACPI_PROCESSOR)

Fixes: 5477fb3b (ACPI / CPPC: Add a CPUFreq driver for use with CPPC)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>