mfd: axp20x: Add self-working mode support for AXP806

The AXP806 can operate in a standalone "self-working" mode, in which it
is also responsible for power control of the overall system. This mode
is similar to the master mode, but the EN/PWRON pin functions as a power
button, instead of a level-triggered enable switch.

This patch adds code checking for the new "x-powers,self-working-mode"
property, and a separate mfd_cell list that includes the power button
(PEK) sub-device.

Signed-off-by: Chen-Yu Tsai <wens@csie.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>