At present we add driver-model MMC devices in the order we find them. The
'alias' order is not honoured.

It is difficult to fix this for the case where we have holes in the
sequence. But for the common case where the devices are numbered from 0
without any gaps, we can add the devices to the internal data structures
in this order.

Signed-off-by: Simon Glass <sjg@chromium.org>