mtd: ubi: Add kconfig to handle RD at attach

Add a new Kconfig flag to optionally turn on
handling read disturb at ubi attach. Turning on
this flag can cause significant delays at attach.

During the first boot the system would not have acquired
a valid time stamp and all the erase times are marked as
zero (since the clock start at zero).
Then on subsequent bootups if the system has an updated
time stamp it would cause all the PEB's to be marked for
scrubbing as this time difference would be much larger
than the time threshold.

Change-Id: I71c1b87b4407774bbb17f8acfdacbca537d87365
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>