ACPI / processor: Update the comments in processor.h

In commit 46ba51ea (ACPI / processor: Introduce ARCH_MIGHT_HAVE_ACPI_PDC),
acpi_processor_set_pdc() was moved to processor_pdc.c, so update
the comments accordingly.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>