firmware: turris-mox-rwtm: Add sysfs documentation

Add sysfs ABI documentation for the sysfs files created by the
turris-mox-rwtm driver.

Link: https://lore.kernel.org/r/20190822014318.19478-4-marek.behun@nic.czSigned-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/ABI/testing/sysfs-firmware-turris-mox-rwtm

+What:		/sys/firmware/turris-mox-rwtm/board_version
+Date:		August 2019
+KernelVersion:	5.4
+Contact:	Marek Behún <marek.behun@nic.cz>
+Description:	(R) Board version burned into eFuses of this Turris Mox board.
+		Format: %i
+
+What:		/sys/firmware/turris-mox-rwtm/mac_address*
+Date:		August 2019
+KernelVersion:	5.4
+Contact:	Marek Behún <marek.behun@nic.cz>
+Description:	(R) MAC addresses burned into eFuses of this Turris Mox board.
+		Format: %pM
+
+What:		/sys/firmware/turris-mox-rwtm/pubkey
+Date:		August 2019
+KernelVersion:	5.4
+Contact:	Marek Behún <marek.behun@nic.cz>
+Description:	(R) ECDSA public key (in pubkey hex compressed form) computed
+		as pair to the ECDSA private key burned into eFuses of this
+		Turris Mox Board.
+		Format: string
+
+What:		/sys/firmware/turris-mox-rwtm/ram_size
+Date:		August 2019
+KernelVersion:	5.4
+Contact:	Marek Behún <marek.behun@nic.cz>
+Description:	(R) RAM size in MiB of this Turris Mox board as was detected
+		during manufacturing and burned into eFuses. Can be 512 or 1024.
+		Format: %i
+
+What:		/sys/firmware/turris-mox-rwtm/serial_number
+Date:		August 2019
+KernelVersion:	5.4
+Contact:	Marek Behún <marek.behun@nic.cz>
+Description:	(R) Serial number burned into eFuses of this Turris Mox device.
+		Format: %016X

MAINTAINERS

@@ -1632,6 +1632,7 @@ W:	http://mox.turris.cz
 S:	Maintained
 F:	Documentation/ABI/testing/debugfs-moxtet
 F:	Documentation/ABI/testing/sysfs-bus-moxtet-devices
+F:	Documentation/ABI/testing/sysfs-firmware-turris-mox-rwtm
 F:	Documentation/devicetree/bindings/bus/moxtet.txt
 F:	Documentation/devicetree/bindings/firmware/cznic,turris-mox-rwtm.txt
 F:	Documentation/devicetree/bindings/gpio/gpio-moxtet.txt