dt-bindings: hwmon: Add adm127x documentation

adm127x are hot-swap controllers that allow a circuit board to be removed from
or inserted into a live backplane. This patch adds the device tree
bindings documentation.

Signed-off-by: Kun Yi <kunyi@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>