gpio/omap: cleanup omap_gpio_mod_init function

With register offsets now defined for respective OMAP versions we can get rid
of cpu_class_* checks. This function now has common initialization code for
all OMAP versions. Initialization specific to OMAP16xx has been moved within
omap16xx_gpio_init().

Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Signed-off-by: Charulatha V <charu@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>