cmd: add serial console support for the cls command

Currently the cls command does not support the serial console

The screen can be cleared in the video uclass, the colored frame buffer
console, and the serial console by sending the same escape sequence.
This reduces the cls command to a single printf() statement on most
boards.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>