[POWERPC] bootwrapper: Make ft_get_phandle() accept and return NULL.

Currently, if ft_get_phandle() is passed NULL it will allocate an entry
for it and return a non-NULL phandle.  This patch makes it simply pass
the NULL through.

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