mfd: twl-core: Collect global variables behind one private structure (global)

Gather the global variables under a single structure and allocate it with
devm_kzalloc(). It is easier to see them and if in the future we try to add
support for multiple instance of twl in the system it is going to be much
simpler.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>