mtd: add BLKPG API based repartition support

mtd: add BLKPG API based repartition support

Add support for mtd repartition based on the block
device BLKPG interface:
BLKPG_ADD_PARTITION - for partition creation;
BLKPG_DEL_PARTITION - for partition delete

The usage is based on BLKPG ioctl called with
struct blkpg_ioctl_arg argument which includes the
reference to struct blkpg_partition discribing the
partition offset and length.

Disadvantage: there is no implementation for mtd
flags control. The flags are always borrowed from
the master device.

Signed-off-by: Roman Tereshonkov <[email protected]>
Signed-off-by: Artem Bityutskiy <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>