mtd: spinand: winbond: Add comment about the maximum frequency

Clarify that Winbond octal capable chips may be clocked at up to 166MHz,
which is their absolute maximum.

No per-operation maximum value (captured with a "0" in the table)
involves that in these cases the maximum frequency of the chip applies,
ie. the one commonly described in the DT.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>