mtd: spi-nor: Add a ->setup() method

nor->params.setup() configures the SPI NOR memory. Useful for SPI NOR
flashes that have peculiarities to the SPI NOR standard, e.g.
different opcodes, specific address calculation, page size, etc.
Right now the only user will be the S3AN chips, but other
manufacturers can implement it if needed.

Move spi_nor_setup() related code in order to avoid a forward
declaration to spi_nor_default_setup().

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