mtd: nand: Add OX820 NAND Support

Add NAND driver to support the Oxford Semiconductor OX820 NAND Controller.
This is a simple memory mapped NAND controller with single chip select and
software ECC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>