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 value and 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)
- value: the reset value written to the reboot 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:

	reboot {
	   compatible = "syscon-reboot";
	   regmap = <&regmapnode>;
	   offset = <0x0>;
	   mask = <0x1>;
	};