pinctrl: intel: merrifield: Add pin config group handlers

Pin config get() and set() handlers for pin groups were previously not
implemented by this driver. The pin_config_group_set() is particularly useful
for applying a common config setting to all pins in a specified group with a
single call, without the caller needing to reference each individual pin by
name.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>