documentation: move mtrr.txt to Doc/x86/ subdir

Move mtrr.txt to the Documentation/x86/ subdirectory.
Add 00-INDEX to the Documentation/x86/ subdirectory.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 03980cb..06f9354 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1131,7 +1131,7 @@
 	  You can safely say Y even if your machine doesn't have MTRRs, you'll
 	  just add about 9 KB to your kernel.
 
-	  See <file:Documentation/mtrr.txt> for more information.
+	  See <file:Documentation/x86/mtrr.txt> for more information.
 
 config MTRR_SANITIZER
 	bool