PPC: E500: Generate device tree on reset

Today we generate the device tree once on machine initialization and then
store the finalized blob in memory to reload it on reset.

This is bad for 2 reasons. First we potentially waste a bunch of RAM for no
good reason, as we have all information required to regenerate the device
tree available anyways.

The second reason is even more important. On machine init when we generate
the device tree for the first time, we don't have all of the devices fully
initialized yet. But the device tree needs to potentially walk devices to
put information about them into the device tree.

Move the generation into a reset function. That way we just generate it new
every time we reset, solving both of the above issues.

Signed-off-by: Alexander Graf <agraf@suse.de>
