sandbox: Compile test device tree when CONFIG_UT_DM is defined

A conflict between the PMIC and unit test work means that the sandbox test
device tree file is no-longer built. Fix this.

Series-to: u-boot
Series-cc: joe, prz

Change-Id: I6616428e05713e5306f848e7dd0a645dedf0934e
Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/Makefile

@@ -1,5 +1,5 @@
 dtb-$(CONFIG_SANDBOX) += sandbox.dtb
-dtb-$(CONFIG_DM_TEST) += test.dtb
+dtb-$(CONFIG_UT_DM) += test.dtb
 
 targets += $(dtb-y)