clk: ux500: Initial support for abx500 clock driver

The abx500 clock driver is a platform driver which will be initialized
during arch init. The platform device shall be added from the ab-core
driver as a mfd child device to maintain correct boot sequence.

Depending on what ab version we use, different clock definitions will
be added.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>