e500: Add support for eTSEC in device tree

This patch adds support to expose eTSEC devices in the dynamically created
guest facing device tree. This allows us to expose eTSEC devices into guests
without changes in the machine file.

Because we can now tell the guest about eTSEC devices this patch allows the
user to specify eTSEC devices via -device at all.

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