OMAP: GPIO: remove duplicate debugfs interface

The generic gpiolib provides a debugfs interface to GPIOs which
provides identical (but nicer looking) data as the OMAP specific one.

This patch completely drops the OMAP specific interface
(/debug/omap_gpio) in favor of using the generic one (/debug/gpio.)

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>