ARM: OMAP2+: AM33XX: Fix race between request_irq and gpio_mask_irq

After random iteration, uart standby using (gpio pin configs) hangs.

Upon deep observation (and lots of debug prints), it was observed that
the GPIO Rising/Falling detect registers were cleared (IRQ disabled)
before system entered standby. Any UART activity (key press) was not
detected.

This registers were properly setup by request_irq call from
am33xx_pm_prepare_late() (initial suspend stage).

However, driver suspend calls (.suspend()) come in later stage and due
to some race condition, gpio_mask_irq() masks/clears above registers.

The fix is to call the standby setup function (which calls request_irq)
at final stage just before the actual suspend call.

This fix was tested by placing the system under standby stress test for
more than 20 Hours.

Signed-off-by: Hebbar Gururaja <gururaja.hebbar@ti.com>