mtd: spi-nor: hisi-sfc: Try to provide some clarity on which SFC we are

The driver is for the HiSilicon FMC (Flash Memory Controller), which
supports SPI NOR in addition other memory technologies, like SPI NAND.

Indeed, the naming in the driver is a little inappropriate, especially
considering that there is already another HiSilicon SPI NOR flash
controller (which I believe the FMC is derived from).

Since we now want to provide software support for this other HiSilicon
controller, update code comments to at least try to make it clear that
this driver is for the FMC.

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>