kconfig: kill off GENERIC_IO option

The GENERIC_IO option is set for every architecture except tile and score
as those define NO_IOMEM. The option only controls visibility of
CONFIG_MTD which doesn't appear to be necessary for any reason, so let's
just remove GENERIC_IO.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: user-mode-linux-user@lists.sourceforge.net
Cc: linux-mtd@lists.infradead.org
Acked-by: Richard Weinberger <richard@nod.at>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>