component: allow missing unbind callback

The component framework reuses the devres managed functions. There is no
need to specify an unbind() callback if the driver only wants to release
the devres managed resources. The bind/unbind is like the probe/remove
pair. The bind/probe is necessary and the unbind/remove is optional.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20200227104547.30085-1-m.felsch@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>