mfd: ab8500-core: Add abx500-clk as an mfd child device

Hierarchically, the abx500-clk shall be considered as a child of the
ab8500 core. The abx500-clk is intiated at arch init and thus the clks
will be available when clients needs them.

Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>