bus: moxtet: Add sysfs and debugfs documentation

Add sysfs ABI documentation for the attribute files module_id and
module_name

Add debugfs ABI documentation for reading input from the shift registers
and reading last written output or write output to the shift registers.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Marek Behún <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

Documentation/ABI/testing/debugfs-moxtet

···
+What:		/sys/kernel/debug/moxtet/input
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <[e-mail]>
+Description:	(R) Read input from the shift registers, in hexadecimal.
+		Returns N+1 bytes, where N is the number of Moxtet connected
+		modules. The first byte is from the CPU board itself.
+		Example: 101214
+			 10: CPU board with SD card
+			 12: 2 = PCIe module, 1 = IRQ not active
+			 14: 4 = Peridot module, 1 = IRQ not active
+
+What:		/sys/kernel/debug/moxtet/output
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <[e-mail]>
+Description:	(RW) Read last written value to the shift registers, in
+		hexadecimal, or write values to the shift registers, also
+		in hexadecimal.
+		Example: 0102
+			 01: 01 was last written, or is to be written, to the
+			     first module's shift register
+			 02: the same for second module

Documentation/ABI/testing/sysfs-bus-moxtet-devices

···
+What:		/sys/bus/moxtet/devices/moxtet-<name>.<addr>/module_description
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <[e-mail]>
+Description:	(R) Moxtet module description. Format: string
+
+What:		/sys/bus/moxtet/devices/moxtet-<name>.<addr>/module_id
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <[e-mail]>
+Description:	(R) Moxtet module ID. Format: %x
+
+What:		/sys/bus/moxtet/devices/moxtet-<name>.<addr>/module_name
+Date:		March 2019
+KernelVersion:	5.3
+Contact:	Marek Behún <[e-mail]>
+Description:	(R) Moxtet module name. Format: string

MAINTAINERS

···
 M:	Marek Behun <[e-mail]>
 W:	http://mox.turris.cz
 S:	Maintained
+F:	Documentation/ABI/testing/debugfs-moxtet
+F:	Documentation/ABI/testing/sysfs-bus-moxtet-devices
 F:	Documentation/devicetree/bindings/bus/moxtet.txt
 F:	include/linux/moxtet.h
 F:	drivers/bus/moxtet.c