devicetree: hwmon: shtc1: add sensirion,shtc1.yaml

Add documentation for the newly added DTS support in the shtc1 driver.
To align with the drivers logic to have high precision by default
a boolean sensirion,low-precision is used to switch to low precision.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200815012227.32538-3-chris.ruehl@gtsys.com.hk
Signed-off-by: Guenter Roeck <linux@roeck-us.net>