x86: video: Add support for CONFIG_CONSOLE_SCROLL_LINES

Some machines are very slow to scroll their displays. To cope with this,
support the CONFIG_CONSOLE_SCROLL_LINES option. Setting this to 5 allows
the display to operate at an acceptable speed by scrolling 5 lines at
a time.

This same option is available for LCDs so when these systems are unified
this code can be unified also.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>