mtd: spinand: Octal DTR support

mtd: spinand: Octal DTR support

Merge series from Miquel Raynal <miquel.raynal@bootlin.com>:

This series adds support for 8D-8D-8D in SPI NAND, which can already be
leveraged without any SPI changes as controllers already have this
support for some SPI NOR devices.

Among the few spi-mem patches, they are needed for building the SPI NAND
changes (especially the ODTR introduction at the end) and therefore an
immutable tag will be needed for merging in the MTD tree (unless all the
series goes through MTD directly ofc).