xen/acpi: Fix Kconfig dependency on CPU_FREQ

The functions: "acpi_processor_*" sound like they depend on CONFIG_ACPI_PROCESSOR
but in reality they are exposed when CONFIG_CPU_FREQ=[y|m]. As such
update the Kconfig to have this dependency and fix compile issues:

ERROR: "acpi_processor_unregister_performance" [drivers/xen/xen-acpi-processor.ko] undefined!
ERROR: "acpi_processor_notify_smm" [drivers/xen/xen-acpi-processor.ko] undefined!
ERROR: "acpi_processor_register_performance" [drivers/xen/xen-acpi-processor.ko] undefined!
ERROR: "acpi_processor_preregister_performance" [drivers/xen/xen-acpi-processor.ko] undefined!

Note: We still need the CONFIG_ACPI
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/Kconfig

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index b1d524a..9424313 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -180,7 +180,7 @@
 
 config XEN_ACPI_PROCESSOR
 	tristate "Xen ACPI processor"
-	depends on XEN && X86 && ACPI_PROCESSOR
+	depends on XEN && X86 && ACPI_PROCESSOR && CPU_FREQ
 	default m
 	help
           This ACPI processor uploads Power Management information to the Xen hypervisor.