Documentation: dtb: lm87: Add hwmon binding documentation

This patch adds lm87 hwmon device tree node documentation.

Signed-off-by: Mahoda Ratnayaka <mahoda.ratnayaka@alliedtelesis.co.nz>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>