Implement the Freescale i.MX31 advanced vectored interrupt controller, at least
to the extent it is used by Linux 3.x

Vectors are not implemented.
Signed-off-by: NPhilip O'Sullivan <philipo@ok-labs.com>
Signed-off-by: NPeter Chubb <peter.chubb@nicta.com.au>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>