mtd: spi-nor: core: Introduce method for RDID op

RDID is used in the core to auto detect the flash, but also by some
manufacturer drivers that contain flashes that support Octal DTR mode,
so that they can read the flash ID after the switch to Octal DTR was made
to test if the switch was successful. Introduce a core method for RDID op
to avoid code duplication.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220420103427.47867-5-tudor.ambarus@microchip.com