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.

Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com>
Reviewed-by: Patrice CHOTARD <patrice.chotard@stericsson.com>
Reviewed-by: Gabriel FERNANDEZ <gabriel.fernandez@stericsson.com>
Reviewed-by: Philippe BEGNIC <philippe.begnic@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>