In the vendor IDE, there's a device family named GW1N-9C (which seems to
mean C revision of GW1N-9), in which the model numbers are all the same
with GW1N-9.
Add an option to nextpnr-gowin to allow manually specified family for
this situation.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>