clk: ux500: Provide u8500_clk with skeleton Device Tree support

The functional components will be added on a per-clock basis.

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