mtd: spi-nor: Move SFDP logic out of the core

It makes the core file a bit smaller and provides better separation
between the SFDP parsing and core logic.

Keep the core.h and sfdp.h definitions private in drivers/mtd/spi-nor/.
Both expose just the definitions that are required by the core and
manufacturer drivers. None of the SPI NOR controller drivers should
include them.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>