microblaze: Add mm/Kconfig to advance menu

mm/Kconfig is getting too big to be in root menu.
Move it to submenu.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/Kconfig

@@ -248,10 +248,10 @@ config MICROBLAZE_64K_PAGES
 
 endchoice
 
-endmenu
-
 source "mm/Kconfig"
 
+endmenu
+
 menu "Executable file formats"
 
 source "fs/Kconfig.binfmt"