kconfig: add a Memory Management options" menu

This moves all the options under a proper menu.

Based on a patch from Randy Dunlap.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

mm/Kconfig

+
+menu "Memory Management options"
+
 config SELECT_MEMORY_MODEL
 	def_bool y
 	depends on ARCH_SELECT_MEMORY_MODEL
@@ -762,3 +765,5 @@ config GUP_BENCHMARK
 
 config ARCH_HAS_PTE_SPECIAL
 	bool
+
+endmenu