clk: versatile: pass a name to ICST clock provider

When we have more than one of these clocks in a system (such as
on the IM-PD1) we need a mechanism to pass a name for the clock.
Refactor to add this as an argument.

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