[POWERPC] bootwrapper: Make ft_get_parent() return a phandle, and NULL if already top-level.

Most of ft_get_parent() is factored out into __ft_get_parent(), which
deals only in internal node pointers.  The ft_get_parent() wrapper
handles phandle conversion in both directions (previously,
ft_get_parent() did not convert its return value).

It also now returns NULL as the parent of the toplevel node, rather than
just returning the toplevel node again (which made it rather useless in
loops).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>