mtd: spi-nor: constify fsl_qspi_devtype_data

All fsl_qspi_devtype_data structures are never modified.
This patch constify them.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Acked-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>