ecp5: Add GSR and SGSR support

[daveshah1]

This adds support for SGSR and GSR for ECP5, and also mirrors Diamond behaviour thus:

• the GSR parameter is set to "DISABLED" when no GSR primitive is used and "ENABLED" otherwise. A non-standard override (* nogsr *) can also be used to override this
• active low async FF resets connected to GSR are pruned

See also YosysHQ/nextpnr#319

cc @whitequark

[whitequark]

Thank you, this looks excellent.