pinctrl: cherryview: add handlers for pin_config_group_get/set

Pin config get/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: Dan O'Donovan <dan@emutex.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>