Documentation/ABI: Add new attribute for mlxreg-io sysfs interfaces

Add documentation for the new attributes for exposing reset causes for
the next type of resets: caused by different watchdog, BIOS reload,
ASIC reset request.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>