sunxi: power: axp818: add support for axp818 driver

AXP818 is rsb based PMIC and used on Allwinner A83T H8 Homlet dev board.
It's registers are different and calculating reg config is different than
that of earlier axp power ICs.

DCDC1, DCDC2, DCDC3 and DCDC5 is implemented at the moment.
all other voltages can be added subsequently.
AXP datasheet is uploaded to wiki:
http://linux-sunxi.org/File:AXP818_datasheet_Revision1.0.pdf



Signed-off-by: Vishnu Patekar <vishnupatekar0510@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>