dt: power: Add docs for generic SYSCON poweroff driver.

Add documentation for generic SYSCON poweroff driver.
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power/reset/syscon-poweroff.txt

+Generic SYSCON mapped register poweroff driver
+
+This is a generic poweroff driver using syscon to map the poweroff register.
+The poweroff is generally performed with a write to the poweroff register
+defined by the register map pointed by syscon reference plus the offset
+with the mask defined in the poweroff node.
+
+Required properties:
+- compatible: should contain "syscon-poweroff"
+- regmap: this is phandle to the register map node
+- offset: offset in the register map for the poweroff register (in bytes)
+- mask: the poweroff value written to the poweroff register (32 bit access)
+
+Default will be little endian mode, 32 bit access only.
+
+Examples:
+
+	poweroff {
+	   compatible = "syscon-poweroff";
+	   regmap = <&regmapnode>;
+	   offset = <0x0>;
+	   mask = <0x7a>;
+	};