LTC3651 and other GPIO chargers

[Ladislav Michl <ladis@linux-mips.org>]

Hi there!

A driver for LTC3651 was recently added to -next
https://patchwork.kernel.org/patch/9717049/
which brings a question whenever we want to add separate driver
(or vendor specific bindings) for ever growing list of similar
chargers. For example those using the same status output lines
are: BQ24232HA, LTC4007, LTM8061, RT9502, LT3651, LT3650, TP4056,
MAX1737...
Others for example BQ24032A and LM3658 (which I have to support)
are using different status encoding, ie. LM3658:

stat1	stat2		Condition
  0	  0	Power-Down, charging is suspended or interrupted
  1	  0	Pre-qualification mode, CC and CV charging
  0	  1	Charge is completed
  1	  1	Bad battery (Safety timer expired), or LDO mode

What about extending gpio-charger instead?
- allow gpio list fdt subnode
  - consider each gpio line to represent bit in a word
- provide per property subnodes
  - each subnode holds a mask and mapping to property values
This way we should be able to cover most chargers providing status
using gpios. Comments welcome and appreciated - those will turn into
implementation.

Thank you,
	ladis