docs: mtd: Update spi nor reference driver

The reference driver no longer exists since commit 50f1242c ("mtd:
fsl-quadspi: Remove the driver as it was replaced by spi-fsl-qspi.c").

Update reference to spi-fsl-qspi.c driver.

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>