clk: ux500: Support for prmcu_rate clock

The prmcu_rate clock is not gateable and has a rate which
only can be fetched.

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