icepll generated PLL module has `apio lint` errors

[zapta]

When generating an ICE40 pll module using the utility icepll, running apio lint results in lint errors.

Upstream report here YosysHQ/icestorm#352

Submitted upstream PR at YosysHQ/icestorm#353

[zapta]

As a workaround, add manually the missing unused signals as shown in the Apio example alhambra-ii/pll.