Documentation: power: reset: Add documentation for generic SYSCON reboot driver

Add documentation for generic SYSCON reboot driver.
Signed-off-by: NFeng Kan <fkan@apm.com>
Signed-off-by: NSebastian Reichel <sre@kernel.org>

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

+Generic SYSCON mapped register reset driver
+
+This is a generic reset driver using syscon to map the reset register.
+The reset is generally performed with a write to the reset register
+defined by the register map pointed by syscon reference plus the offset
+with the mask defined in the reboot node.
+
+Required properties:
+- compatible: should contain "syscon-reboot"
+- regmap: this is phandle to the register map node
+- offset: offset in the register map for the reboot register (in bytes)
+- mask: the reset value written to the reboot register (32 bit access)
+
+Default will be little endian mode, 32 bit access only.
+
+Examples:
+
+	reboot {
+	   compatible = "syscon-reboot";
+	   regmap = <&regmapnode>;
+	   offset = <0x0>;
+	   mask = <0x1>;
+	};