mfd: axp20x: Add a cell for the power button part of the, axp288 PMICs

This patch adds the mfd cell info for axp288 power key device.

Signed-off-by: Borun Fu <borun.fu@linux.intel.com>
Signed-off-by: Fei Yang <fei.yang@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>