mtd: spi-nor: Stop prefixing generic functions with a manufacturer name

Replace the manufacturer prefix by something describing more precisely
what those functions do.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
[tudor.ambarus@microchip.com: prepend spi_nor_ to all modified methods.]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>