mfd: mp2629: Add support for mps battery charger

mp2629 is a highly-integrated switching-mode battery charge management
device for single-cell Li-ion or Li-polymer battery.

Add MFD core enables chip access for ADC driver for battery readings,
and a power supply battery-charger driver

Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>