cpufreq: CPPC: Fix build error without CONFIG_ACPI_CPPC_CPUFREQ_FIE

If CONFIG_ACPI_CPPC_CPUFREQ_FIE is not set, building fails:

drivers/cpufreq/cppc_cpufreq.c: In function ‘populate_efficiency_class’:
drivers/cpufreq/cppc_cpufreq.c:584:2: error: ‘cppc_cpufreq_driver’ undeclared (first use in this function); did you mean ‘cpufreq_driver’?
  cppc_cpufreq_driver.register_em = cppc_cpufreq_register_em;
  ^~~~~~~~~~~~~~~~~~~
  cpufreq_driver

Make declare of cppc_cpufreq_driver out of CONFIG_ACPI_CPPC_CPUFREQ_FIE
to fix this.

Fixes: 740fcdc2 ("cpufreq: CPPC: Register EM based on efficiency class information")
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>