hw/ppc/e500.c: Avoid hardcoding parent device in create_devtree_etsec()

The "platform" node is available through data->node, so use that instead
of making assumptions about the parent device.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230125130024.158721-4-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>