Wide bus support.

This adds 4-bit bus support to the MMC layer. It is designed to 
(hopefully) be compatible with a future 4-bit MMC implementation. This 
is done by seperating the three different instances of bus width defines:

* Protocol definition: SD_BUS_WIDTH_x
* SCR contents: SD_SCR_BUS_WIDTH_x
* Host mode: MMC_BUS_WIDTH_x

They have the same values atm but drivers should not rely on this. 
MMC_BUS_WIDTH_x is not meant to be SD specific.

The MMC layer changes bus width when a card is selected. This is because 
the SD spec says that a card is only required to keep a certain bus 
width as long as it's selected.

Layers further up do not need to know which mode the host/card is in. 
They will only see a change in speed.