mtd: add writebufsize field to mtd_info struct

This field will be used to indicate the write buffer size
of the MTD device. UBI will set it's minimal I/O unit size
(min_io_size) to the indicated write buffer size. By this
change we intend to fix failed recovery of UBIFS partitions
we currently observe on NOR flash when mounting the partition
after unclean unmount.

Currently the min_io_size is set to mtd->writesize (which is 1
byte for NOR flash). But flash programming is often done from
prepared write buffer containing multiple bytes and is performed
in one programming operation which could be interrupted by a power
cut or a system reset causing corrupted (partially written) areas
in a flash sector. Knowing the size of potentially corrupted areas
UBIFS scanning and recovery algorithms are able to perform
successful recovery.

In case of NOR flash minimal I/O size must be equal to the
maximal size of the write buffer used by embedded flash
programming algorithm. In case of NAND flash mtd->writebufsize
should be equivalent to mtd->writesize.

The subsequent patches will add mtd->writebufsize initialization
where needed.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>