ecp5_gsr - ECP5: handle GSR

yosys> help ecp5_gsr

ecp5_gsr [options] [selection]

Trim active low async resets connected to GSR and resolve GSR parameter,
if a GSR or SGSR primitive is used in the design.

If any cell has the GSR parameter set to "AUTO", this will be resolved
to "ENABLED" if a GSR primitive is present and the (* nogsr *) attribute
is not set, otherwise it will be resolved to "DISABLED".