mtd: spi-nor: Add support for en25qh64

The Eon EN25QH64 is a 64 Mbit SPI NOR flash memory chip found
on recent wireless routers. Its 32, 128 and 256 Mbit siblings
are already supported.

Tested on a COMFAST CF-E120A v3 router board.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>