clk: ux500: Support is_prepared callback for clk-prcmu

To be able to gate unused prcmu clocks from the clk_disable_unused sequence,
clk-prcmu now implements the is_prepared callback.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>