mfd: ab8500-core: Rework MFD sub-device initialisation structures

Here we're separating Battery Management devices into their own
structure, removing the common device structure & redistribute them
amongst the individual platform structs and completing the population
of them.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>