pinctrl: improve debugfs for strict controllers

If we know we are using a strict pin controller (one that cannot
mix device functions+group use and GPIO) we can be a bit more
specific in debugfs, just print either device-function-group
or GPIO consumer for the pin. Let's do that to be helpful.

Cc: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>