A barebox environment oftree node may reference its storage by a phandle
to a partition node under a MCI node. barebox will then call the
device's detect method to detect the card if this hasn't happened
before. Out of 17 MCI drivers, 8 host drivers already implement
a detect method, which just calls mci_detect_card.

Provide a generic implementation that does the same.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>