doc: Update docs for device tree in SPL, TPL

Make a few small updates to indicate that device tree can be used in SPL
and TPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.fdt-control b/doc/README.fdt-control
index 88c0469..d6ab7bf 100644
--- a/doc/README.fdt-control
+++ b/doc/README.fdt-control
@@ -174,6 +174,16 @@
     $ make DEVICE_TREE=<dts-file-name>
 
 
+Relocation, SPL and TPL
+-----------------------
+
+U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
+
+The full device tree is available to U-Boot proper, but normally only a subset
+(or none at all) is available to TPL and SPL. See 'Pre-Relocation Support' and
+'SPL Support' in doc/driver-model/README.txt for more details.
+
+
 Limitations
 -----------