rp2040: fix hang on startup on Twotrees SK1 toolhead board

[grafalex82]

After upgrading to recent Klipper, the Twotrees SK1 toolhead's microcontroller does not boot normally. In most of the cases it hangs during startup, or falls into HardFault exception.

Using the onboard LED connected to GPIO23, setting it to on/off in different places of the initialization code it was found that the issue happens around configuring the PLLs. Moreover, adding a small delay at the end of pll_setup() function fixes the issue, and the board boots normally.

Notes:

• I understand that adding a raw delay looks suspicious - I am open for ideas how to make this in a more elegant way. In the meantime I tried using udelay() from generic/armcm_timer.c, but it does not compile for rp2040.
• I also tried to use while (!(pll->cs & PLL_CS_LOCK_BITS)) similar to the loop above, but it does not fix the issue.
• I checked with the datasheet, but it does not indicate necessity of any waits here, but adding waits definitely improves the situation.

[JamesH1978]

Thank you for submitting a PR, pleas refer to point 3 in "What to expect in a review" in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md and provide a signed off by line.

Thanks James

[amken3d]

@grafalex82 I had similar issues on some custom boards that I have made. In my case, it turned out to be due to the flash not booting correctly. I had created a fix for that issue on this branch of my fork https://github.com/amken3d/klipper/tree/rp2040-add-support-for-additional-flash-chips. You can try it and see if it helps you.

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