mtd: spi-nor: Add the concept of SPI NOR manufacturer driver

Declare a spi_nor_manufacturer struct and add basic building blocks to
move manufacturer specific code outside of the core.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>