macintosh: Add missing of_node_get() in do_attach()

We need a of_node_get() for of_find_compatible_node() to keep refcount
balance.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220622061652.4095330-1-windhl@126.com