hwmon: (ltc2978) device tree bindings documentation

Add device tree bindings documentation for ltc2978.

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>
[Guenter Roeck: Minor correction of 'compatible' example]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>