lib: correct description of CONFIG_SYS_FDT_PAD

CONFIG_SYS_FDT_PAD defines the number of unused bytes added to a
device-tree and not the total size.

Fixes: 40ed7be4af52 ("Convert CONFIG_SYS_FDT_PAD to Kconfig")
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index 1a683de..a21b337 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -1008,14 +1008,14 @@
 	  This enables the FDT library (libfdt) overlay support.
 
 config SYS_FDT_PAD
-	hex "Maximum size of the FDT memory area passeed to the OS"
+	hex "Free space added to device-tree before booting"
 	depends on OF_LIBFDT
 	default 0x13000 if FMAN_ENET || QE || U_QE
 	default 0x3000
 	help
-	  During OS boot, we allocate a region of memory within the bootmap
-	  for the FDT. This is the size that we will expand the FDT that we
-	  are using will be extended to be, in bytes.
+	  The operating system may need a free area at the end of the device-
+	  tree for fix-ups. This setting defines by how many bytes U-Boot
+	  extends the device-tree before booting.
 
 config SPL_OF_LIBFDT
 	bool "Enable the FDT library for SPL"