clk: versatile-icst: refactor to allocate regmap separately

Break out the registration function so it creates a regmap and
pass to the setup function, so the latter can be shared with
a device tree probe function that already has a regmap.

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>