Back out part-implemented DDR support for eMMC devices
Support for DDR signalling was further advanced with regard to eMMC devices than it was either for SD devices or controllers. This meant that if an eMMC device supports DDR signalling and the controller's capabilities reported that it also supported DDR (irrespective of the fact that driver support for DDR at the controller end is still absent), we ended up with a msimatch between the signalling at the controller and the device. The practical upshot of this was that eMMC devices don't complete enumeration on 4th-generation Raspberry Pis.
As a stop-gap solution until DDR support for controllers is written, wrap the MMC DDR support in #if 0 statements.