gpio: sysfs: Enforce character device

If users select sysfs support they get the character device
as well so that end-users cannot complain that they
"only have sysfs on my system". They should have the
character device at all times.

If someone is in so dire need of stripping out the
character device while still enabling the sysfs ABI they
can very well patch the kernel.

Also only show this obsolete option to expert users.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201110142724.14760-1-linus.walleij@linaro.org