mfd: Remove confusing ab8500-i2c file and merge into ab8500-core

ab8500-i2c is used as core code to register the ab8500 device.
After allocating ab8500 memory, it immediately calls into
ab8500-core where the real initialisation takes place. This
patch moves all core registration and memory allocation into
the true ab8500-core file and removes ab8500-i2c completely.

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