mtd: spi-nor: add spi_nor_controller_ops_{read_reg, write_reg, erase}()

They are thin wrappers around
nor->controller_ops->{read_reg,write_reg,erase}(). In a future commit
DTR support will be added. These ops can not be supported by the
controller_ops hooks and these helpers will make it easier to reject
those calls.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201005153138.6437-3-p.yadav@ti.com