[WIP] sysfs entries for axp20x mfd driver

For mainline kernel 4.4+

Main purpose of this patch is creating sysfs interface to get information from AXP202/209 PMU until proper power driver is implemented in mainline kernel.

➜  ~  % ls /sys/power/axp_pmu/{ac,vbus,battery,charger,pmu,control,ocv_curve}
/sys/power/axp_pmu/ocv_curve

/sys/power/axp_pmu/ac:
amperage  connected  used  voltage

/sys/power/axp_pmu/battery:
amperage  capacity  charge  charging  connected  power  ts_voltage  voltage

/sys/power/axp_pmu/charger:
amperage  cell_activation  charging  low_power

/sys/power/axp_pmu/control:
battery_rdc         disable_fuel_gauge    set_vbus_direct_mode
charge_rtc_battery  reset_charge_counter

/sys/power/axp_pmu/pmu:
overheat  temp  voltage

/sys/power/axp_pmu/vbus:
amperage  connected  strong  used  voltage
➜  ~  %

Properties table

Subsystem	Attribute	Type	Unit	AXP Reg	Notes
ac	voltage	integer	µV	56h:57h
ac	amperage	integer	µA	58h:59h
ac	connected	boolean	-	00h[7]
ac	used	boolean	-	00h[6]
vbus	voltage	integer	µV	5Ah:5Bh
vbus	amperage	integer	µA	5Ch:5Dh
vbus	connected	boolean	-	00h[5]
vbus	used	boolean	-	00h[4]
vbus	strong	boolean	-	00h[3]
battery	voltage	integer	µV	78h:79h
battery	amperage	integer	µA	7Ch:7Dh	Discharge current
battery	ts_voltage	integer	µV	62h:63h
battery	power	integer	µW	70h:72h
battery	capacity	integer	%	B9h[6:0]
battery	charge	integer	µAh	B0h:B7h
battery	connected	boolean	-	01h[5]
battery	charging	boolean	-	00h[7]	Direction of current
pmu	temp	integer	m°C	5Eh:5Fh
pmu	voltage	integer	µV	7Eh:7Fh
pmu	overheat	boolean	-	01h[7]
charger	amperage	integer	µA	7Ah:7Bh	Charge current
charger	charging	boolean	-	01h[6]
charger	cell_activation	boolean	-	01h[3]	Mode for charging deeply discharged cell
charger	low_power	boolean	-	01h[2]	Not enough input power
control	set_vbus_direct_mode	boolean	-	30h[6]
control	reset_charge_counter	boolean	-	B8h[5]
control	charge_rtc_battery	boolean	-	35h[7]
control	disable_fuel_gauge	boolean	-	B9h[7]
control	battery_rdc	integer	µΩ	BAh:BBh
-	ocv_curve	byte[]	%	C0h:CFh

Manual battery calibration tool

Usage: axp20x-calibrate-battery [-h | -p | -e | -l <file> | -s <file>]
Arguments:
-h:             Show this help message and exit
-p:             Print configuration
-e:             Edit configuration
-s <file>:      Save OCV table to <file>
-l <file>:      Load and apply OCV table from <file>

Editing (-e) and loading (-l) requires root privileges