Add a qemu_irq_split() function which allows a board to wire a single
GPIO output up to two GPIO inputs. This is needed for realview boards,
where the MMC card status is visible both in a system register and
via a PL061 GPIO module.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>