gpio: omap: Allow building as a loadable module

We currently get all kinds of errors building the omap gpio driver
as a module starting with:

undefined reference to `omap2_gpio_resume_after_idle'
undefined reference to `omap2_gpio_prepare_for_idle'
...

Let's fix the issue by adding inline functions to the header.
Note that we can now also remove the two unused functions for
omap_set_gpio_debounce and omap_set_gpio_debounce_time.

Then doing rmmod on the module produces further warnings
because of missing exit related functions. Let's add those.

And finally, we can make the Kconfig entry just a tristate
option that's selected for omaps.

Cc: Javier Martinez Canillas <javier@dowhile0.org>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@linaro.org>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>