power/supply: unify DT documentation

Currently the power supply device tree documentation is spread
in .../bindings/power_supply and .../bindings/power. This
unifies the files for chargers and battery fuel gauges in
.../bindings/power/supply and the ones for system reset/shutdown
in .../bindings/power/reset (same structure as used for the
drivers itself).

Signed-off-by: Sebastian Reichel <sre@kernel.org>