pl061: fix wrong calculation of GPIOMIS register

The masked interrupt status register should be the state of the interrupt
after masking.
There should be a logical AND instead of a logical OR between the
interrupt status and the interrupt mask.

Signed-off-by: Victor CLEMENT <victor.clement@openwide.fr>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1433154824-6927-1-git-send-email-victor.clement@openwide.fr
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>