fdt: Allow libfdt in TPL

In some cases (e.g. sandbox with verified boot) it is useful to support
libfdt in TPL. Update the Kconfig to handle this.

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

lib/Kconfig

@@ -353,6 +353,16 @@ config SPL_OF_LIBFDT
 	  particular compatible nodes. The library operates on a flattened
 	  version of the device tree.
 
+config TPL_OF_LIBFDT
+	bool "Enable the FDT library for TPL"
+	default y if TPL_OF_CONTROL
+	help
+	  This enables the FDT library (libfdt). It provides functions for
+	  accessing binary device tree images in memory, such as adding and
+	  removing nodes and properties, scanning through the tree and finding
+	  particular compatible nodes. The library operates on a flattened
+	  version of the device tree.
+
 config FDT_FIXUP_PARTITIONS
 	bool "overwrite MTD partitions in DTS through defined in 'mtdparts'"
 	depends on OF_LIBFDT