clk: ux500: Add Device Tree support for the PRCC Kernel clock

This patch enables clocks to be specified from Device Tree via phandles
to the "prcc-kernel-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>