[ARM] Kirkwood: fail the probe if internal RTC does not work

Having a RTC that doesn't maintain proper time across a reboot is one
thing.  But a RTC that doesn't work at all and only causes timeouts is
another.

Tested-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>