cmd: add clear screen 'cls' command

Add common clear screen command for configurations
CONFIG_DM_VIDEO, CONFIG_LCD and CONFIG_CFB_CONSOLE.

Remove the existing cls command implementation from
lcd.c code and activate the command for all boards
enabling CONFIG_LCD for compatibility reasons.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Patrick.Delaunay <patrick.delaunay@free.fr>