fdt: Document the fact that dtc is now built

This documentation is out of date now that U-Boot builds dtc
automatically. Update it.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/README.fdt-control

@@ -56,7 +56,11 @@ In case you are wondering, OF stands for Open Firmware.
 Tools
 -----
 
-To use this feature you will need to get the device tree compiler here:
+To use this feature you will need to get the device tree compiler. This is
+provided by U-Boot automatically. If you have a system version of dtc
+(typically in the 'device-tree-compiler' package), it is currently not used.
+
+If you want to build your own dtc, it is kept here:
 
 	git://git.kernel.org/pub/scm/utils/dtc/dtc.git