mtd: ubi: Add sysfs entry to force all pebs' scan

A given eraseblock can be read many times or not at all.
We do not have the account of such eraseblocks that have
not been accessed since a pre-defined threshold interval.
By means of scanning the entire flash device it is possible to identify
all such eraseblocks.

Add a sysfs entry to force scan on all the PEBs on demand basis.

The sysfs entry would be available under /sys/class/ubi/ubiX/peb_scan
 - echo 1 to this entry would trigger the scan, ignored if in progress
 - On reading returns the scan status (1 = Scan executing, 0 = Scan not
   executing)

Change-Id: Ifeb966808db7b382f7aeb009bbf10b8a68f87f70
Signed-off-by: Pratibhasagar V <pratibha@codeaurora.org>
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>