dt-bindings: mfd: axp20x: Add "self-working" mode for AXP806

The AXP806 has three operation modes:

  - master mode: The PMIC is the first or only AXP PMIC in the system,
		 but is not in charge of power management, i.e. only
		 provides regulator functions.

  - slave mode: The PMIC is the second AXP PMIC in the system, chained
		to the first, or master, one.

  - self-working mode: The PMIC is the only AXP PMIC in the system, and
		       is in charge of power sequencing.

The functional differences between these modes can be found in the
"Control and Operation" chapter of the AXP806 (in Chinese) and AXP805
(in English) datasheets. These include how the PMIC responds to external
signals, whether it takes an external voltage reference or uses its own,
and whether the EN/PWRON pin functions as an enable switch or power button.

We already support both slave and master mode. This patch adds a property
for describing the self-working mode, and reworks the description for
the mode properties.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Icenowy Zheng <icenowy@aosc.io>
Tested-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>