pinctrl: Show selected function and group in pinmux-pins debugfs

Until recently, the pinctrl pinmux-pins debugfs file displayed the
selected function for each owned pin. This feature was removed during
restructing in support of recent API rework. This change restoreds this
feature, and also displays the group that the function was selected on,
in case a pin is a member of multiple groups.

Based on work by: Linus Walleij <linus.walleij@linaro.org>

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>