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

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/power/power-controller.txt

-* 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;
 }