of-platdata: improve documentation

Improve some things in the documentation of OF_PLATDATA that I found
while porting socfgpa_gen5 to it.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Kconfig b/dts/Kconfig
index 8917f42..3e85914 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -265,8 +265,7 @@
 
 	  This option works by generating C structure declarations for each
 	  compatible string, then adding platform data and U_BOOT_DEVICE
-	  declarations for each node. See README.platdata for more
-	  information.
+	  declarations for each node. See of-plat.txt for more information.
 
 config TPL_OF_PLATDATA
 	bool "Generate platform data for use in TPL"
@@ -287,8 +286,7 @@
 
 	  This option works by generating C structure declarations for each
 	  compatible string, then adding platform data and U_BOOT_DEVICE
-	  declarations for each node. See README.platdata for more
-	  information.
+	  declarations for each node. See of-plat.txt for more information.
 
 endmenu