clk: ux500: Add armss clk and fixup smp_twd clk for u8500

The new armss clk is a prcmu_scalable_rate clk which represents
the ARMSS clk. This then makes it possible to convert the smp_twd
clk to a fixed factor clock type, using a fixed divider of 2 and
with the armss clk as parent.

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>