kernel: time: Add udelay_test module to validate udelay

Create a module that allows udelay() to be executed to ensure that
it is delaying at least as long as requested (with a little bit of
error allowed).

There are some configurations which don't have reliably udelay
due to using a loop delay with cpufreq changes which should use
a counter time based delay instead.  This test aims to identify
those configurations where timing is unreliable.

Signed-off-by: David Riley <davidriley@chromium.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>

lib/Kconfig.debug

@@ -1649,6 +1649,15 @@ config TEST_BPF
 
 	  If unsure, say N.
 
+config TEST_UDELAY
+	tristate "udelay test driver"
+	default n
+	help
+	  This builds the "udelay_test" module that helps to make sure
+	  that udelay() is working properly.
+
+	  If unsure, say N.
+
 source "samples/Kconfig"
 
 source "lib/Kconfig.kgdb"