dt-bindings: hwmon: Add ltc2947 documentation

Document the LTC2947 device devicetree bindings.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191011114853.159327-2-nuno.sa@analog.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>