For these primitives, Gowin decided to use a different option for
describing ports—directly in the module header, i.e.
``` verilog
module ADC(input CLK);
```
instead of
``` verilog
module ADC(CLK);
input CLK;
```
Since this one-time parser becomes too confusing, it is easier to simply
add ADC descriptions as they are from a separate file, especially since
these primitives are only available in the GW5A series.
Test:
``` shell
yosys -p "read_verilog top.v; synth_gowin -json top-synth.json -family gw5a"
```
The old version of Yosys simply won't compile the design due to the lack
of port descriptions, while the new version will compile without errors.
Signed-off-by: YRabbit <rabbit@yrabbit.cyou>
