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>

doc/README.fdt-control

@@ -174,6 +174,16 @@ After board configuration is done, fdt supported u-boot can be build in two ways
     $ 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
 -----------
 

doc/driver-model/README.txt

@@ -832,7 +832,8 @@ Pre-Relocation Support
 For pre-relocation we simply call the driver model init function. Only
 drivers marked with DM_FLAG_PRE_RELOC or the device tree
 'u-boot,dm-pre-reloc' flag are initialised prior to relocation. This helps
-to reduce the driver model overhead.
+to reduce the driver model overhead. This flag applies to SPL and TPL as
+well, if device tree is enabled there.
 
 It is possible to limit this to specific relocation steps, by using
 the more specialized 'u-boot,dm-spl' and 'u-boot,dm-tpl' flags