rpi: always scroll by 10 lines

Scrolling the simple framebuffer is really slow in Raspberry Pi to the
point it delays the boot by a second or two and makes longer output
inconvenient to follow (printenv, md).

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>