[POWERPC] Add mpc52xx_find_and_map_path(), refactor utility functions

Add helper routine mpc52xx_find_and_map_path(). Extract common code to
mpc52xx_map_node() and refactor mpc52xx_find_and_map().

Signed-off-by: Jan Wrobel <wrr@semihalf.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>