dragon-nx-32: wire up the probing and mpi logic

dragon-nx-32: wire up the probing and mpi logic

First cut at this.

We make the IDE base a variable
Allow for a platform private header for IDE (needed for stuff like ide_base)
Issue select/deselect commands passing the drive number

This in theory also gives us all that is needed for multiple IDE controllers,
while not paying a cost for them on simple platforms (they can define the
selectors into nothingness)