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/20190812161118.21476-4-marek.behun@nic.cz


Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>