pinctrl: intel: Make offset to interrupt status register configurable

Some GPIO blocks have the interrupt status (GPI_IS) offset different
than it normally is, so make it configurable. If no offset is specified
we use the default.

While there remove two unused constants from the core driver.

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