mtd: spi-nor: Introduce 'struct spi_nor_controller_ops'

Move all SPI NOR controller driver specific ops in a dedicated
structure. 'struct spi_nor' becomes lighter.

Use size_t for lengths in 'int (*write_reg)()' and 'int (*read_reg)()'.
Rename wite/read_buf to buf, the name of the functions are
suggestive enough. Constify buf in int (*write_reg). Comply with these
changes in the SPI NOR controller drivers.

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