[ARM] pxa: fix issue of muxed GPIO irq_chip functions touching non-muxed GPIOs

pxa_gpio_irq_type() and pxa_unmask_muxed_gpio() will touch non-muxed GPIOs
(0 and 1 on PXA2xx/PXA3xx) bits in GRERx and GFERx, which is incorrect.
Actually, only those bits should get updated if the corresponding bits are
set in c->irq_mask as well. Fix this by updating only those relevant bits.

Reported-and-tested-by: Daniel Ribeiro <drwyrm@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>