Implement TI AM335x's SDHC reset quirk.  Beaglebone SDHC works now!

On the AM335x, we first must wait for the controller to acknowledge
the reset; then we can wait for the reset to complete.

I believe this quirk also applies to the OMAP4 ES, but I don't have
one of those to test and we don't seem to have an obvious conditional
for it anyway.

This quirk may work for controllers that don't require it too, but I
am nervous about doing it by default because if we miss the reset
acknowledgement, then we'll just time out even though everything is
really hunky-dory.

Also, for all sdhc, don't bother writing 0 in sdhc_soft_reset while
waiting for the reset to complete; there is no need.

ok matt

(riastradh)