mfd: axp20x: Change battery cell name to fuel gauge

Name changes to the battery cell structure to a
more generic cell type: fuel gauge.

Signed-off-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Acked-By: Sebastian Reichel <sre@kernel.org>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>