clk: versatile: pass a parent to the ICST clock

clk: versatile: pass a parent to the ICST clock

As we want to actually define the parent frequency in the device
tree for the ICST clocks, modify the clock registration function
to take a parent argument.

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