cores/video/VideoTimingGenerator: Allow passing custom dict as default_video_timings.

Allow only listing the classical video timings in the core and let user provide the timings
specific to other configurations.

litex/soc/cores/video.py

@@ -152,7 +152,8 @@ video_data_layout = [
 
 class VideoTimingGenerator(Module, AutoCSR):
     def __init__(self, default_video_timings="800x600@60Hz"):
-        # Check / Get Video Timings.
+        # Check / Get Video Timings (can be str or dict)
+        if isinstance(default_video_timings, str):
             try:
                 self.video_timings = vt = video_timings[default_video_timings]
             except KeyError:
@@ -160,6 +161,8 @@ class VideoTimingGenerator(Module, AutoCSR):
                 for video_timing in video_timings.keys():
                     msg.append(f" - {video_timing} / {video_timings[video_timing]['pix_clk']/1e6:3.2f}MHz.")
                 raise ValueError("\n".join(msg))
+        else:
+            self.video_timings = vt = default_video_timings
 
         # MMAP Control/Status Registers.
         self._enable      = CSRStorage(reset=1)