dt-uart: Clarify log messages at init time.

dt-uart: Clarify log messages at init time.

- Don't log at all if console=dtuart (the default) was not present, in
  that case the user has asked for something else, no need for every
  other driver to tell them this.
- Use "dtuart" in all other messages, rather than just "console" or
  "uart".
- Be more explicit if we are exiting because dtuart= wasn't given.
- Log the options which we've parsed.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>