[POWERPC] mv64x60: Remove device tree absolute path references

Replace several device node absolute path lookups in the mv64x60
bootwrapper code with lookups by compatible or device_type
properties.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>