cpufreq: brcmstb-avs-cpufreq: add debugfs support

In order to aid debugging, we add a debugfs interface to the driver
that allows direct interaction with the AVS co-processor.

The debugfs interface provides a means for reading all and writing some
of the mailbox registers directly from the shell prompt and enables a
user to execute the communications protocol between ARM CPU and AVS CPU
step-by-step.

This interface should be used for debugging purposes only.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
diff --git a/drivers/cpufreq/Kconfig.arm b/drivers/cpufreq/Kconfig.arm
index 04f7862..920c469 100644
--- a/drivers/cpufreq/Kconfig.arm
+++ b/drivers/cpufreq/Kconfig.arm
@@ -23,6 +23,16 @@
 
 	  Say Y, if you have a Broadcom SoC with AVS support for DFS or DVFS.
 
+config ARM_BRCMSTB_AVS_CPUFREQ_DEBUG
+	bool "Broadcom STB AVS CPUfreq driver sysfs debug capability"
+	depends on ARM_BRCMSTB_AVS_CPUFREQ
+	help
+	  Enabling this option turns on debug support via sysfs under
+	  /sys/kernel/debug/brcmstb-avs-cpufreq. It is possible to read all and
+	  write some AVS mailbox registers through sysfs entries.
+
+	  If in doubt, say N.
+
 config ARM_DT_BL_CPUFREQ
 	tristate "Generic probing via DT for ARM big LITTLE CPUfreq driver"
 	depends on ARM_BIG_LITTLE_CPUFREQ && OF