This adds the BCM2835 GPIO controller.

It currently implements:
- The 54 GPIOs as outputs (qemu_irq)
- The SD controller selection via alternate function of GPIOs 48-53
Signed-off-by: NClement Deschamps <clement.deschamps@antfield.fr>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1488293711-14195-4-git-send-email-peter.maydell@linaro.org
Message-id: 20170224164021.9066-4-clement.deschamps@antfield.fr
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>