test: check u-boot properties in SPL device tree

Add a test to check the management of the U-boot relocation properties
for device tree SPL generation (fdtgrep result) and platdata:
- 'dm-pre-proper' and 'dm-tpl' not included in SPL
- 'dm-pre-reloc' and 'dm-spl' included in SPL

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>