ice40: PLLs can't conflict with themselves
ice40: PLLs can't conflict with themselves

Fixes error building testcase from #145

Signed-off-by: David Shah <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@ds0.me>

SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr