[POWERPC] Add for_each_child_of_node() helper for iterating over child nodes

Add for_each_child_of_node() to encapsulate the common idiom of
iterating over the children of a device_node.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>