genirq/timings: Add selftest for circular array

Due to the complexity of the code and the difficulty to debug it, add some
selftests to the framework in order to spot issues or regression at boot
time when the runtime testing is enabled for this subsystem.

This tests the circular buffer at the limits and validates:
 - the encoding / decoding of the values
 - the macro to browse the irq timings circular buffer
 - the function to push data in the circular buffer

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: andriy.shevchenko@linux.intel.com
Link: https://lkml.kernel.org/r/20190527205521.12091-7-daniel.lezcano@linaro.org

lib/Kconfig.debug

@@ -1858,6 +1858,14 @@ config TEST_PARMAN
 
 	  If unsure, say N.
 
+config TEST_IRQ_TIMINGS
+	bool "IRQ timings selftest"
+	depends on IRQ_TIMINGS
+	help
+	  Enable this option to test the irq timings code on boot.
+
+	  If unsure, say N.
+
 config TEST_LKM
 	tristate "Test module loading with 'hello world' module"
 	depends on m