cpufreq: acpi-cpufreq: make Intel/AMD MSR access, io port access static

These frequency register read/write operations' implementations for the
given processor (Intel/AMD MSR access or I/O port access) are only used
internally in acpi-cpufreq, so make them static.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>