apm821xx: wndr4700: add preliminary drivetemp sensor

apm821xx: wndr4700: add preliminary drivetemp sensor

This patch prepares the WNDR4700 to use the HDD sensor for
the thermal zone. While the kernel's thermal.txt device-tree
binding documentation files talks about supporting multiple
sensors for a zone. This sadly is NOT the case. Even the most
current upstream kernels (5.6-rc) supports just >one< sensor
per zone: (driver/base/of-thermal.c:886)
| * REVIST: for now, the thermal framework supports only
| * one sensor per thermal zone. Thus, we are considering
| * only the first two values as slope and offset.

I do hope that this warning will prevent others wasteing time
on trying to figure out why their multi-sensor thermal-zones
definitions are not working as specified.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>