mtd: spi-nor: Trim what is exposed in spi-nor.h

The SPI NOR controllers drivers must not be able to use structures that
are meant just for the SPI NOR core.

struct spi_nor_flash_parameter is filled at run-time with info gathered
from flash_info, manufacturer and sfdp data. struct spi_nor_flash_parameter
should be opaque to the SPI NOR controller drivers, make sure it is.

spi_nor_option_flags, spi_nor_read_command, spi_nor_pp_command,
spi_nor_read_command_index and spi_nor_pp_command_index are defined for the
core use, make sure they are opaque to the SPI NOR controller drivers.

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