dt-bindings: hwmon: allow specifying channels for tmp421

Add binding description for the per temperature channel configuration
like labels and n-factor.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/d1721a50efc0f88728614b5b07c0d9332ae4971c.1634206677.git.krzysztof.adamski@nokia.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>