gpio: omap: Add missing PM ops for suspend

We've had the legacy platform code take care of suspend for us but
this no longer is the case when probed without legacy mode with
ti-sysc. We need to configure PM ops like standard Linux device
drivers do.

As we still have some SoCs booting also the legacy mode, we need to
add omap_gpio_suspend() and omap_gpio_resume(), and check for the
is_suspended flag to avoid legacy _od_suspend_noirq() calling them
on an already suspended GPIO instance.

Once we have no SoCs booting in legacy mode, we can just switch to
using the standard PM ops with pm_runtime_force_suspend() and
pm_runtime_force_resume().

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>