pinctrl: pinconf: Allow groups to be configured via debugfs

The function pinconf_dbg_config_write() currently only supports configuring
a pin configuration mapping via the debugfs. Allow group mappings to also
be configured via the debugfs.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>