mtd: spi-nor: Add support for EN25Q80A

This adds support for the EON EN25Q80A, a 8Mb SPI NOR chip.
It is used on i.MX6 boards by Kontron Electronics GmbH
(N60xx, N61xx).
It was only tested with a single data line connected, by writing and
reading random data with dd.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>