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 value and 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)
- value: the poweroff value written to the poweroff register (32 bit access)

Optional properties:
- mask: update only the register bits defined by the mask (32 bit)

Legacy usage:
If a node doesn't contain a value property but contains a mask property, the
mask property is used as the value.

Default will be little endian mode, 32 bit access only.

Examples:

	poweroff {
	   compatible = "syscon-poweroff";
	   regmap = <&regmapnode>;
	   offset = <0x0>;
	   mask = <0x7a>;
	};