mtd: nand: Add OX820 NAND hardware dependency

The oxnas NAND driver is only needed for a specific platform, do
not propose it on other platforms unless build-testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 66859249 ("mtd: nand: Add OX820 NAND Support")
Cc: Rob Herring <robh@kernel.org>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>