Add a dhrystone benchmark command

Drystone provides a convenient sanity check that the CPU is running at full
speed. Add this as a command which can be enabled as needed.

Note: I investigated using Coremark for this but there was a license
agreement and I could not work out if it was GPL-compatible.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index c98d3997..972ac17 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -50,6 +50,8 @@
 	help
 	  This library provides pseudo-random number generator functions.
 
+source lib/dhry/Kconfig
+
 source lib/rsa/Kconfig
 
 menu "Hashing Support"