mmc: am654_sdhci: Implement workaround for card detect

mmc: am654_sdhci: Implement workaround for card detect

The 4 bit MMC controllers have an internal debounce for the SDCD line
with a debounce delay of 1 second. Therefore, after clocks to the IP are
enabled, software has to wait for this time before it can power on the
controller.

Add a deferred_probe() callback which polls on sdcd for a maximum of 2 seconds
before switching on power to the controller or (in the case of no card)
returning a ENOMEDIUM. This pushes the 1 second wait time to when the
card is actually needed rather than at every probe() making sure that
users who don't insert an SD card in the slot don't have to wait such a
long time.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>