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

At present the SATA command code includes both the command-processing code
and the core SATA 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:
- All CONFIG option are referenced from the non-command code
- The concept of a 'current SATA 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>