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

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>;

	};