mtd: spi-nor: add Dual and Quad read mode support to some flash devices

These devices are used on OpenPOWER systems. The SPI_NOR_DUAL_READ
flags is added for the Aspeed SoCs which do not support QUAD reads.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>