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.czSigned-off-by: NMarek Behún <marek.behun@nic.cz>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>