mtd: remove bcmring NAND driver

This driver is being removed as part of the cleanup of the bcmring
SoC from mainline as it is no longer maintained.

Signed-off-by: Christian Daudt <csd@broadcom.com>
Reviewed-by: Jiandong Zheng <jdzheng@broadcom.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>