ACPI processor: make /proc/acpi/processor/*/throttle depends on CONFIG_ACPI_PROCFS

As a feature that would only be used when system is overheating,
the processor t-state control should not be exported to user space.
Make /proc/acpi/processor/*/throttle depends on CONFIG_ACPI_PROCFS,
which is cleared by default.
And we will remove this I/F in 2.6.38.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>