pinctrl: core: Show pin numbers for the controllers with base = 0

The commit f1b206cf ("pinctrl: core: print gpio in pins debugfs file")
enabled GPIO pin number and label in debugfs for pin controller. However,
it limited that feature to the chips where base is positive number. This,
in particular, excluded chips where base is 0 for the historical or backward
compatibility reasons. Refactor the code to include the latter as well.

Fixes: f1b206cf

 ("pinctrl: core: print gpio in pins debugfs file")
Cc: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Drew Fustini <drew@beagleboard.org>
Reviewed-by: Drew Fustini <drew@beagleboard.org>
Link: https://lore.kernel.org/r/20210415130356.15885-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>