mtd: spinand: winbond: Rename DTR variants

So far all the chips supported in the driver apparently have support for
the same kind of operation (typically, single, dual and quad). The
future introduction of W35N chips will change that as these chips only
support single and octal modes. Let's rename the variants accordingly to
make these future additions more understandable.

Acked-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>