examples: change all clocks for ch32v003 to use internal oscillator by default

[lokuciejewski]

Some examples for ch32v003 use SYSCLK_FREQ_48MHZ_HSE as their clock source which prevents them from working on any board that does not have the external oscillator installed.

I changed all examples to use SYSCLK_FREQ_48MHZ_HSI and added explicit clock initialisation in pwm example. Now the examples should work on any ch32v003 board.

[Codetector1374]

Not sure if this is the right thing to do though, given that most examples are meant to work on specific boards / pinout etc. Changing the oscillator to HSI is not the only thing you need to change to have examples work on a different board than what is intended.

[lokuciejewski]

Oh, my bad, I guess I must have missed that those are for specific boards. But I think it would be good to make the examples as board-independent as possible - after all, the hal is for the specific microcontroller, not any specific board.

[Codetector1374]

Well I think being board specific is kind of unavoidable for embedded stuff. It can potentially be worked around with having configuration file etc but I think that actually does more damage than good, because it would add complexity to what is supposed to be a "simple example" use of the HAL.

The HAL is for the chip, not the board designed around it.

[lokuciejewski]

Yeah, I agree the examples should not be overcomplicated and I am not proposing to add any board-specific configuration files. Changing to internal oscillator shouldn't break the compatibility with these boards - it would only enable those examples to work on any ch32v003 chip.
But if you still think this is unnecessary, then I can just close this PR without merging.