mtd: spinand: Enhance the logic when picking a variant

mtd: spinand: Enhance the logic when picking a variant

Currently the best variant picked in the first one in the list provided
in the manufacturer driver. This worked well while all operations where
performed at the same speed, but with the introduction of DTR transfers
this no longer works correctly.

Let's continue iterating over all the alternatives, even if we find a
match, keeping a reference over the theoretically fastest
operation. Only at the end we can tell which variant is the best.

This logic happening only once at boot.

The patch is based on linux commit
666c299be696 (mtd: spinand: Enhance the logic when picking a variant)
created by Miquel Raynal <miquel.raynal@bootlin.com>

The code was a bit restricted in the functionality since not all
required functionality is supported in the u-boot.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>