Allow users to enable specific exception events via debugfs.

The bits enabled by the driver ee_drv_ctrl are separated from the bits
enabled by the user ee_usr_ctrl. The control mask ee_mask_ctrl is the
logical-or of those two. A mutex is needed to ensure that the masks match
what was written to the device.

Link: https://lore.kernel.org/r/20210209062437.6954-4-adrian.hunter@intel.comAcked-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>