Add support for Waveshare Modbus Relays

[ep1cman]

Waveshare only implement the ability to query the status of all relays (https://www.waveshare.com/wiki/Modbus_RTU_Relay#Read_States_of_Relays) not just one, so a modification to the existing drivers is required.

Checklist

• [x] Documentation for the feature
• [ ] Tests for the feature

• [x] The arguments and description in doc/configuration.rst have been updated

• [ ] Add a section on how to use the feature to doc/usage.rst

• [ ] Add a section on how to use the feature to doc/development.rst

• [x] PR has been tested

• [ ] Man pages have been regenerated