The driver used to always add the mmc device as it's child even
it no card was detected. Add a function that will detect if the
card is present or not and that will attach/detach the mmc device.
The function is either call on attach (as we won't have the interrupt
fired) or from two taskqueues. The first taskqueue will directly call
the function when the sdcard was present and is now removed and the other
one will delay a bit the attach when we didn't had a card and now have one.
This is mostly based on comments from the sdhci driver where it describe
a situation when the CD pin is detected before the others pins are connected.
MFC after: 1 month
87d4212b1d