uboot-imx

log: Add a console driver

It is useful to display log messages on the console. Add a simple driver
to handle this.

Note that this driver outputs to the console, which may be serial or
video. It does not specifically select serial output.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>