gowin: add an option to manually specify family
gowin: add an option to manually specify family

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 <cf9dc3e94649e480c789a6b538c4d95e82540aa4@aosc.io>

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