feat(smartplug_usb): add basic Green Power GP-SP1U model
This is very basic, exposing only switches and timers, but it is following
the same standard dps layout, and since all the config and sensor entites are
already hidden: unavailable, the config will be a perfect fit, with the caveat
that this device does not report any dps in initial poll, so timers may also
end up disabled by default until the user manually reenables them.
Issue #6078
Jason Rumney