[POWERPC] Update documentation for of_find_node_by_type()

The documentation for of_find_node_by_type() incorrectly refers to the
"name" parameter - it should be "type".

Also the behaviour when from == NULL is not really documented, fix that.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>