pinctrl: cherryview: Introduce helpers to IO with common registers

Pin control device and effectively the single community in it has
a set of common registers. It's good to have a helpers to IO on them.

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