[POWERPC] bootwrapper: Add ft_root_node().

Clean up some of the open-coded data structure references by providing a
function to return a pointer to the tree's root node.  This is only used
in high-level functions trying to access the root of the tree, not in
low-level code that is actually manipulating the data structure.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>