We lack a way to instantiate a fixed clock while specifying a parent.
This is used in the at91 clock code sync with upstream in a later
commit, so prepare by porting clk_register_fixed_rate.

It's based on the Linux commit of the same name with the difference that
it doesn't use (and thus doesn't require) a struct device_d * as first
parameter.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>