gpiolib: devprop: Warn if gpio-line-names is too long

Some DT authors (including myself) have messed up the length of
gpio-line-names and made it longer than it should be. Add a warning here
so that developers can figure out that they've messed up their DT and
should fix it.

Cc: Alexandru M Stan <amstan@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>