dm: ide: Separate the non-command code into its own file

At present the IDE command code includes both the command-processing code
and the core IDE functions and data structures.

Separate the latter into its own file, adding functions as needed to avoid
the command code accessing data structures directly.

With this commit:
- Most CONFIG option are referenced from the non-command code
- The concept of a 'current IDE device' is confined to the command code

This will make it easier to convert this code to driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>