gpio/omap: remove suspend/resume callbacks

Both omap_gpio_suspend() and omap_gpio_resume() does programming
of wakeup_en register.
_gpio_rmw(base, bank->regs->wkup_en, 0xffffffff, 0);
_gpio_rmw(base, bank->regs->wkup_en, bank->context.wake_en, 1);

This is redundant in omap_gpio_suspend() because wakeup_en
register automatically gets initialized in _set_gpio_wakeup()
and set_gpio_trigger() while being called either from
chip.irq_set_wake() or chip.irq_set_type().

This is also redundant in omap_gpio_resume() because wakeup_en
register is programmed in omap_gpio_restore_context() called
which is called from runtime resume callback.

Cc: Kevin Hilman <khilman@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Cousson, Benoit <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Govindraj.R <govindraj.raja@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>