mps2-an511: Fix wiring of UART overflow interrupt lines

Fix an error that meant we were wiring every UART's overflow
interrupts into the same inputs 0 and 1 of the OR gate,
rather than giving each its own input.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1505232834-20890-1-git-send-email-peter.maydell@linaro.org
