docs: ABI: testing: Document the OCC hwmon FFDC binary interface

Add documentation for the new binary sysfs that will dump the SBEFIFO
FFDC.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20211019205307.36946-4-eajames@linux.ibm.com


Signed-off-by: Joel Stanley <joel@jms.id.au>