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

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>;

	};