mtd: spi-nor: amend the rdsr dummy cycles documentation

The rdsr dummy cycles are only used by the 8d-8d-8d mode. Mention that
in the documentation.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220418112837.2792242-1-michael@walle.cc