Set up stdio earlier when using driver model

Since driver model registers itself with the stdio subsystem, and we
want to avoid delayed registration and other complexity associated with
the current serial console, move the stdio subsystem init earlier when
driver model is used for serial.

This simplifies the implementation. Should there be any problems with
this approach they can be dealt with as boards are converted over to
use driver model for serial.

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