mfd/axp20x: add support for fuel gauge cell

Register definitions and platform data structure
for fuel gauge cell devices.
Signed-off-by: NTodd Brandt <todd.e.brandt@intel.com>
Acked-by: NJacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>