fdt: Allow indicating a node is for U-Boot proper only

This add missing parts for previous commit 06f94461
("fdt: Allow indicating a node is for U-Boot proper only")

At present it is not possible to specify that a node should be used before
relocation (in U-Boot proper) without it also ending up in SPL and TPL
device trees. Add a new "u-boot,dm-pre-proper" boolean property for this.

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