Provide a new function sdbus_reparent_card() in sd core for reparenting
a card from a SDBus to another one.

This function is required by the raspi platform, where the two SD
controllers can be dynamically switched.

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: 1488293711-14195-3-git-send-email-peter.maydell@linaro.org
Message-id: 20170224164021.9066-3-clement.deschamps@antfield.fr
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: added a doc comment to the header file; changed to
 use new behaviour of qdev_set_parent_bus()]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>