This adds support for interpreting the input and output bits of one
device on Moxtet bus as GPIOs.
This is needed for example by the SFP cage module of Turris Mox.

Link: https://lore.kernel.org/r/20190812161118.21476-5-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>