mtd: spi-nor: Improve Kconfig help text for SPI_FSL_QUADSPI

The current "We only connect the NOR to this controller now." text
is not very clear, so explain it better by saying that generic SPI
is not supported by SPI_FSL_QUADSPI and only SPI NOR is.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>