mmc: core: Add MMC BKOPS statistics and debugfs ability to print them

The BKOPS statistics are used for BKOPS unit tests and APT tests
to determine test success or failure.
The BKOPS statstics provide the following information:
The number of times BKOPS were issued according to its severity level
The number of times manual BKOPS was started/stopped (HPI)
The number of times auto BKOPS was enabled/disabled

In order to enable and reset the statistics:
echo 1 > /sys/kernel/debug/mmc0/mmc0:0001/bkops_stats
In order to disable the statistics:
echo 0 > /sys/kernel/debug/mmc0/mmc0:0001/bkops_stats
In order to view the statistics:
cat /sys/kernel/debug/mmc0/mmc0:0001/bkops_stats

Change-Id: Ib84319aedfb49dc022bc27efbda842a5db38c7e9
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>