arm: Init the debug UART

At present we don't init the debug UART in the generic ARM code, but
instead leave it to individual machines to handle. This is not the
way it is supposed to work.

Add the required init to the crt files. This ensures that the UART
is available as early as possible and that the announcement appears
when it should, if enabled.

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