gpio: intel-mid: lock IRQs when starting them

This uses the new API for tagging GPIO lines as in use by
IRQs. This enforces a few semantic checks on how the underlying
GPIO line is used.

ChangeLog v1->v2:
- Explicitly call the - empty - mask/unmask functions from the
  startup/shutdown hooks. These are currently empty, but maybe
  they will not be that forever, so better be safe than sorry.

Acked-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>