TI sdhci driver improvements, mostly related to fdt data...

Use the published compatible strings (our own invention, "ti,mmchs" is
still accepted as well, for now).

Don't blindly turn on 8-bit bus mode, because even though the conroller
supports it, the board has to be wired appropriately as well.  Use the
published property (bus-width=<n>) and honor all the valid values (1,4,8).

The eMMC device on a Beaglebone Black is wired for 8-bit, update the dts.

The mmchs controller can inherently do both 1.8v and 3.0v on the first
device and 1.8v only on other devices, unless an external transceiver is
used.  Set the voltage automatically for the first device and honor
the published fdt property (ti,dualvolt) for other devices.

Thanks go to Ilya Bakulin for figuring out the voltage compatibility stuff.
The ability to do 8-bit implies 4-bit capability too.  Rearrange the cases
and add a fallthrough comment to make that happen.