Remove the OMAP1 #ifdef and MPUIO special case for _clear_gpio_irqbank()

The MPUIOs do not need a register access to ack/clear the IRQ status,
since reading the IRQ status clears it.  In addition, the MPUIO
irq_chip has an empty ack method, so _clear_gpio_irqbank() is never
used for MPUIOs.
Signed-off-by: Kevin Hilman <khilman@ti.com>