mtd: ubi: Fill read disturb statistics

Fill in eraseblock statistics needed for read disturb decision making:
1. read counter: incremented at each read operation
		 reset at each erase operation
    Saved only as part of the meta data in RAM
    On attach: if fastmap data is missing a default value is assigned
2. last erase time stamp: updated at each erase operation.
   Saved as part of PEB OOB info on flash
   On attach: if fastmap data is missing retrieved from PEB EC Header
   on NAND.
   If fastmap data is corrupted a default value is assigned.

The above parameters are saved as part of the fastmap data.

Change-Id: I298ecd8f5d0849cb044fafa2854f234b553c8b16
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>