PPC: Drop initial ESCC mapping

We are mapping ESCC to a static (incorrect) address on machine init. This
overlaps with our vram, rendering the screen barely usable.

Since openBIOS is clever enough to map ESCC to where it needs to be, we can
just drop that invalid map and everyone's happy.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
