clk: versatile-icst: convert to use regmap

Instead of passing around register bases, pass around a regmap
in this driver. This refactoring make things so much easier when
we later want to manage an ICST that is part of a syscon.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>