pinctrl: pinconf-generic: Add flag to print arguments

When dumping pinconf information in debugfs, config arguments are only
printed when a unit is present and the argument is != 0. For parameters
like the slew rate, this does not work. The slew rate uses a driver
specific format for the argument, i.e. 0 can be a valid argument and a
unit is not provided for it.
For that reason, add a flag to enable printing the argument instead of
inferring it from the presence of a unit and the value of the argument.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>