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>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index c221125..4c4dba0 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -316,7 +316,7 @@
 	  family of SOCs.
 
 config GPIO_OMAP
-	bool "TI OMAP GPIO support" if COMPILE_TEST && !ARCH_OMAP2PLUS
+	tristate "TI OMAP GPIO support" if ARCH_OMAP2PLUS || COMPILE_TEST
 	default y if ARCH_OMAP
 	depends on ARM
 	select GENERIC_IRQ_CHIP