[ARM] pxa: make GPIO IRQ code less dependent on the internal IRQs

by:

1. introduce dedicated pxa_{mask,unmask}_low_gpio()

2. remove set_irq_chip(IRQ_GPIO_2_x, ...) which has already been
   initialized in pxa_init_irq()

3. introduce dedicated pxa_init_gpio_set_wake()

Signed-off-by: eric miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>