[POWERPC] #address-cells & #size-cells properties are not inherited

Fix error in booting-without-of.txt that indicates that a node can inherit
its #address-cells and #size-cells definitions from its parent's parent.
This is not correct.

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