mtd: spi-nor: cadence-quadspi: write upto 8-bytes data in STIG mode

cadence-quadspi controller allows upto eight bytes of data to
be written in software Triggered Instruction generator (STIG) mode
of operation. Lower 4 bytes are written through writedatalower and
upper 4 bytes by writedataupper register.

This patch allows all the 8 bytes to be written.

Signed-off-by: Purna Chandra Mandal <purna.chandra.mandal@intel.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>