mtd: ubi: Read disturb infrastructure

The need for performing read disturb is determined according to new
statistics collected per eraseblock:
- read counter: incremented at each read operation
                reset at each erase
- last erase time stamp: updated at each erase

This patch adds the infrastructure for the above statistics

Change-Id: I8ff3cf0f391028e5d9d4c9f17e66435a7fcd08be
Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>