Documentation: dtb: xgene: Add hwmon dts binding documentation

This patch adds the APM X-Gene hwmon device tree node documentation.

Signed-off-by: Hoan Tran <hotran@apm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>