PPC: mpc8544ds: Span initial TLB entry over as much RAM as we need

PPC: mpc8544ds: Span initial TLB entry over as much RAM as we need

The initial TLB entry is supposed to help us run the guest -kernel payload.
This means the guest needs to be able to access its own memory, the initrd
memory and the device tree.

So far we only statically reserved a TLB entry from [0;256M[. This patch
fixes it to span from [0;dt_end[, allowing the guest payload to access
everything initially.

Reported-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>