clk: versatile: respect parent rate in ICST clock

If the ICST clock has a parent, respect the rate of the parent
when calculating the clock frequency. As this involves modifying
the ICST parameter struct, make a cloned copy (the divisor
arrays should be safe) so we can update the .ref field.

Do not define the reference clock on the Integrator as we have
the reference clock from the device tree. Keep it everywhere
else.

Cc: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>