[ARM] pxa: make pxa_gpio_irq_type() processor generic

The main issue here is that pxa3xx does not have GAFRx registers,
access directly to these registers should be avoided for pxa3xx:

1. introduce __gpio_is_occupied() to indicate the GAFRx and GPDRx
   registers are already configured on pxa{25x,27x} while returns
   0 always on pxa3xx

2. pxa_gpio_mode(gpio | GPIO_IN) is replaced directly with assign-
   ment of GPDRx, the side effect of this change is that the pin
   _must_ be configured before use, pxa_gpio_irq_type() will not
   change the pin to GPIO, as this restriction is sane, esp. with
   the new MFP framework

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