pinctrl: intel: Define maximum pad number in the group

Instead of using hard coded magic number here and there,
define maximum pad number in the group in newly added
INTEL_PINCTRL_MAX_GPP_SIZE.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>