dt-bindings: mtd: ubi-volume: allow UBI volumes to provide NVMEM

UBI volumes may be used to contain NVMEM bits, typically device MAC
addresses or wireless radio calibration data.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>