control_allocator: allow for only 2 swash plate servos

This is required to support fixed pitch propeller helicopters that have
no collective but only cyclic pitch with two degrees of freedom and
hence only two servos.

The amount of thrust in the body z axis is then controlled using the
motor speed which makes particularly sense on coaxial helicopters
that need to control yaw with changing motor speeds already.

src/modules/control_allocator/module.yaml

@@ -416,6 +416,7 @@ parameters:
                 short: Number of swash plates servos
             type: enum
             values:
+                2: '2'
                 3: '3'
                 4: '4'
             default: 3