[Patch] fbcon: i want fbcon soft scrollback feature come back

[xuhuijie]

This commit 50145474f6ef(fbcon: remove soft scrollback code) remove soft scrollback in
fbcon. So the shift+PageDown and shift+PageUp is missing. But PageUp is a vary important
feature when system panic or reset. I can get log by PageUp before, but now there is no
way to get. Especially on the server system, we always use bmc to control computer.
So I hope the community can add this feature back.