gpio/omap: further cleanup using wkup_en register

Wakeup enable register offset initialized according to OMAP versions
during device registration. Use this to avoid version checks.
Starting with OMAP4, legacy registers should not be used in combination
with the updated regsiters. Use wkup_en register consistently for
all SoCs wherever applicable.

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>