[POWERPC] bootwrapper: Refactor ft_get_prop() into internal and external functions.

The property searching part of ft_get_prop is factored out into an
internal __ft_get_prop() which does not deal with phandles and does not
copy the property data.  ft_get_prop() is then a wrapper that does the
phandle translation and copying.

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