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/dhry/Kconfig b/lib/dhry/Kconfig
new file mode 100644
index 0000000..641b806
--- /dev/null
+++ b/lib/dhry/Kconfig
@@ -0,0 +1,7 @@
+config CMD_DHRYSTONE
+	bool "Support the 'dhry' command to run the dhrystone benchmark"
+	help
+	  Dhrystone is an old benchmark in the public domain that gives a
+	  rough idea of CPU performance. This enables a 'dhry' command
+	  which runs this benchmark within U-Boot and reports the performance.
+	  The number of 'Dhrystone MIPS' is also reported.