clk: ux500: Provide a look-up for the ARMSS clock

The ARMSS clock is used by the newly DT enabled CPUFreq driver.

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