clk: ux500: Add Device Tree support for the RTC clock

This patch enables the RTC fixed frequency clock to be specified from
Device Tree via phandles to the "rtc32k-clock" node.

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>