Fix STM32 + SoftwareSerial compile (BTT002)

[sjasonsmith]

Description

Follow-up to earlier commit which allowed STM32 HAL timer values to be overridden in board pins files (#17805).

That change did not work when using SoftwareSerial with TMC drivers, because HAL.cpp required one of the constants which was moved to timers.cpp.

I have added helper which allows HAL_init to set the software serial interrupt priority without having to actually know the priority value.

Benefits

Allows HAL/STM32 builds which use SoftwareSerial.

I do not currently have hardware set up to test this myself. I'm hoping that @GhostlyCrowd or @thisiskeithb will be able to try it out, since they both encountered this in the attached issue.

Related Issues

#17823

[sjasonsmith]

I'm not positive if my change to the Malyan LCD code is the correct fix for that, I just want to get these checks able to run for my STM32 change.

[thisiskeithb]

Works great here on a BTT002 build. Thanks for the patch!

[GhostlyCrowd]

I can confirm this PR fixes compiling on the SKR Pro 1.1 and software serial for the TMC 2209's is working as expected.

[Fabi0San]

It works, SKR Pro, 2209, auto-fan.
Thank you @sjasonsmith

[sjasonsmith]

I rebased this on top of the ExtUI fix, to get it back out of this PR.

[thinkyhead]

Noice! Force push is encouraged for the PR maintainers. As long as you remember to git merge from origin beforehand, in case someone added a commit or two.

[sjasonsmith]

Noice! Force push is encouraged for the PR maintainers. As long as you remember to git merge from origin beforehand, in case someone added a commit or two.

I guess I cut that one a little bit close if you were modifying it as I rebased it.