omap 5912 osk: GPIO updates

Start cleaning up GPIO handling for OMAP5912 OSK board:

 - Initialize GPIOs using the cross-platform calls, not the old
   OMAP-private ones.

 - Move touchscreen setup out of ads7846 code into board-specfic
   setup code, where it belongs.

This doesn't depend on the patches to update OMAP to use the
gpiolib implementation framework.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>