dm: Allow driver model tests only for sandbox

The GPIO tests require the sandbox GPIO driver, so cannot be run on other
platforms. Similarly for the 'dm test' command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 4e9afe6..c0f2135 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -15,4 +15,6 @@
 # subsystem you must add sandbox tests here.
 obj-$(CONFIG_DM_TEST) += core.o
 obj-$(CONFIG_DM_TEST) += ut.o
+ifneq ($(CONFIG_SANDBOX),)
 obj-$(CONFIG_DM_GPIO) += gpio.o
+endif