MAINTAINERS: take over Kconfig maintainership

I have recently picked up Kconfig patches to my tree without any
declaration.  Making it official now.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -7600,8 +7600,10 @@ F:	mm/kasan/
 F:	scripts/Makefile.kasan
 
 KCONFIG
+M:	Masahiro Yamada <yamada.masahiro@socionext.com>
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git kconfig
 L:	linux-kbuild@vger.kernel.org
-S:	Orphan
+S:	Maintained
 F:	Documentation/kbuild/kconfig-language.txt
 F:	scripts/kconfig/