omap: GPIO: Fix OMAP4 GPIO reg access issues

Access to some of the OMAP4 GPIO registers are not properly handled.
This patch fixes it.

This patch is tested on 3430SDP and 4430SDP boards

Signed-off-by: Charulatha V <charu@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>