mtd: spi-nor: core: Add vdbg msg for spi_nor_erase_multi_sectors()

Useful when debugging non-uniform erase.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210306095002.22983-3-tudor.ambarus@microchip.com