bcm2835: add sdhost and gpio controllers

This adds the bcm2835_sdhost and bcm2835_gpio to the BCM2835 platform.

For supporting the SD controller selection (alternate function of GPIOs
48-53), the bcm2835_gpio now exposes an sdbus.
It also has a link to both the sdbus of sdhci and sdhost controllers,
and the card is reparented from one bus to another when the alternate
function of GPIOs 48-53 is modified.

Signed-off-by: Clement Deschamps <clement.deschamps@antfield.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1488293711-14195-5-git-send-email-peter.maydell@linaro.org
Message-id: 20170224164021.9066-5-clement.deschamps@antfield.fr
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>