Free up some RAM, and go faster

[MichaelBell]

Two unrelated configuration tweaks:

• Move the LWIP buffers to PSRAM. This seems to be working reliably for me, but definitely warrants some more testing. Providing it's stable, it gets us back to a sane amount of available internal RAM, so we shouldn't keep hitting the limit.
• Increase the default clock rate to 200MHz, which will give >60Hz refresh in both full and half res modes while not being too far above spec.

[MichaelBell]

As noted elsewhere, it's possible the 200MHz default clock change caused me some problems flashing, but now it seems to be working again so I'm not sure what the cause was.

[Gadgetoid]

Going to YOLO this one.

I still don't know how likely our PSRAM hacks are to get merged upstream but it wouldn't be the first time I've tended to a downstream hack to keep our boards going.

Maybe this is another opportunity to pitch for optional board-specific linker scripts.