mtd: spi-nor: sfdp: save a copy of the SFDP data

Due to possible mode switching to 8D-8D-8D, it might not be possible to
read the SFDP after the initial probe. To be able to dump the SFDP via
sysfs afterwards, make a complete copy of it.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>