Fix for "Supplied part name is invalid" GW5AST-LV138PG484AC1/I0

[mlapaj]

I have bought Tang Mega 138k. It contains GW5AST-LV138PG484AC1/I0 FPGA IC (I have prepared tang_mega_138k board definition and I'd like to contribute it separately).

When compiling blinky example

I see following error:
Traceback (most recent call last):
File "/home/user/Mega138k/1_Blinky/./blinky.py", line 25, in
platform = TangMega138kPlatform(toolchain='Gowin')
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/oss-cad-suite/lib/python3.11/site-packages/amaranth/vendor/_gowin.py", line 473, in init
self.parse_part()
File "/oss-cad-suite/lib/python3.11/site-packages/amaranth/vendor/_gowin.py", line 215, in parse_part
raise ValueError("Supplied part name is invalid")
ValueError: Supplied part name is invalid

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.28%. Comparing base (ff44683) to head (65bf9f7).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1652      +/-   ##
==========================================
+ Coverage   91.23%   91.28%   +0.04%     
==========================================
  Files          44       44              
  Lines       11402    11517     +115     
  Branches     2224     2242      +18     
==========================================
+ Hits        10403    10513     +110     
- Misses        836      842       +6     
+ Partials      163      162       -1     

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[whitequark]

Thanks, the fix looks reasonable. Please squash the commits and use the following commit message, then I'll merge the PR:

vendor._gowin: add support for GW5AST-LV138PG484AC1/I0.