dt-bindings: Update documentation for "system-power-controller" and fix misspellings

* Generic Poweroff capability

* Generic system power control capability

Power-management integrated circuits or miscellaneous harware components are

sometimes able to control the system power. The device driver associated to these

components might needs to define poweroff capability, which tells to the kernel

how to switch off the system. The corresponding driver must have the standard

property "poweroff-source" in its device node. This property marks the device as

able to shutdown the system. In order to test if this property is found

programmatically, use the helper function "of_system_has_poweroff_source" from

of.h .

Power-management integrated circuits or miscellaneous hardware components are

sometimes able to control the system power. The device driver associated with these

components might need to define this capability, which tells the kernel that

it can be used to switch off the system. The corresponding device must have the

standard property "system-power-controller" in its device node. This property

marks the device as able to control the system power. In order to test if this

property is found programmatically, use the helper function

"of_device_is_system_power_controller" from of.h .

Example:

act8846: act8846@5 {

	 compatible = "active-semi,act8846";

	 status = "okay";

	 poweroff-source;

	 system-power-controller;

}