kconfig: re-add menu_get_parent_menu() that returns parent menu

This helper returns the parent menu, or NULL if there is no parent.
The main difference from the previous version is that it always returns
the parent menu even when the given argument is itself a menu.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>