gpio: exar: Allow IO port access

It's possible that PCI device can provide an IO port resource for
the device. regmap MMIO currently uses MMIO by default. With an
additional flag we enable support for IO port accesses.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>