Fix gptm32 implementation for ch32v20x chips that support it

[marti157]

The following chips have a 32-bit GPTM (on TIM5):

• ch32v203rbt6
• ch32v208cbu6
• ch32v208gbu6
• ch32v208rbt6
• ch32v208wbu6

When building for these chips, there is an error due to TimerBits::Bits32 not being conditionally compiled.

Let me know if there's a better way to fix this other than adding a cfg flag, thanks

[andelf]

Thanks for the fix

[andelf]

I reworked it a little:

• Use chip names as feature flags of special peripherals for now. (I might need to refer stm32-data in the future for a better solution)
• Add CH32V208 to the examples directory, at least make it builds

[andelf]

This is already merged by commits. The "Closed" state is not correct.