Fixes: bbe0df9

I assumed all chipidea devices actually have a clock. This is true
when looking at the device tree, but in barebox not all i.MX SoCs
use the clocks provided by the device tree. Most instead use
clkdev_add_physbase() to attach a clocks to devices. For these the
chipidea device does not have a clock.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>