mtd: spi-nor: intel-spi: Explicitly mark the driver as dangerous in Kconfig

The driver is not meant for normal users at all but instead such users
who really know what they are doing and are able to build their own
kernel to enable it. Mark both driver Kconfig entries as dangerous to
make sure the driver is not accidentally enabled without understanding
possible consequences in doing so.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>