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>