ARM/clk: move the ICST library to drivers/clk

This moves the ICST clock divider helper library from
arch/arm/common to drivers/clk/versatile so it is maintained
with the other clock drivers.

We keep the structure as a helper library intact and do not
fuse it with the clk-icst.c Versatile ICST clock driver: there
may be other users out there that need to use this library for
their clocking, and then it will be helpful to keep the
library contained. (The icst.[c|h] files could just be moved
to drivers/clk/lib or a similar location to share the library.)

Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>