of_clk_add_provider() may be called for devices added via
add_generic_device(). There is no OF node in this case, leading to a crash
when debug logs are enabled.

This affects various i.MX CPUs, which add imx*-ccm clock devices using
add_generic_device().

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>