mmc: mmc-debugfs: add error statistics

This change adds support to allow user-space to query the number of
occurrence of the different type of errors that low-level mmc driver
has encountered. These stats can be read/cleared via debugfs.

Change-Id: I7625ff5145c542e05d2d3515a213512a5273e39c
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>