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/Makefile b/lib/Makefile
index 97ed398..ca72187 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -15,6 +15,8 @@
 obj-$(CONFIG_TIZEN) += tizen/
 obj-$(CONFIG_OF_LIBFDT) += libfdt/
 obj-$(CONFIG_FIT) += libfdt/
+obj-$(CONFIG_FIT) += libfdt/
+obj-$(CONFIG_CMD_DHRYSTONE) += dhry/
 
 obj-$(CONFIG_AES) += aes.o
 obj-$(CONFIG_USB_TTY) += circbuf.o