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

Documentation/ABI/testing/sysfs-bus-platform-devices-occ-hwmon

+What:		/sys/bus/platform/devices/occ-hwmon.X/ffdc
+KernelVersion:	5.15
+Contact:	eajames@linux.ibm.com
+Description:
+		Contains the First Failure Data Capture from the SBEFIFO
+		hardware, if there is any from a previous transfer. Otherwise,
+		the file is empty. The data is cleared when it's been
+		completely read by a user. As the name suggests, only the data
+		from the first error is saved, until it's cleared upon read. The OCC hwmon driver, running on
+		a Baseboard Management Controller (BMC), communicates with
+		POWER9 and up processors over the Self-Boot Engine (SBE) FIFO.
+		In many error conditions, the SBEFIFO will return error data
+		indicating the type of error and system state, etc.