dm: Make driver model available before board_init()

For some boards board_init() will change GPIOs, so we need to have driver
model available before then. Adjust the board init to arrange this, but
enable it for driver model only, just to be safe.

This does create additional #ifdef logic, but it is safer than trying to
make a pervasive change which may cause some boards to break.

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