Added support to STM32F103x6 (Low Density) MCUs, fixed UART bug

[exentio]

Description

Types of Changes

• Added support to STM32F103x6 Low Density MCUs, which have become "popular" due to some chinese manufacturers silently swapping STM32F103x8 on Bluepill boards. Despite the halved flash and RAM, they're still quite useful, especially for prototyping. See Support for new bluepill stm32f103c6t6a with 32k flash size #15297.
• Fixed [Bug] uart.c has wrong arguments #15675.

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

#15297
#15675

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[sigprof]

This should probably be split into two separate PRs.

For the UART change you may argue that it can go directly into master as an obvious bugfix. The existing code does not even compile, but apparently it was never actually used by any of the keyboards tested by CI, so the bug went through unnoticed.

The F103x6 change should go into the develop branch, as it requires a corresponding ChibiOS update. But actually it might need to wait for either a proper 20.3.5 stable release, or a major ChibiOS upgrade (the F103x6 USB fix just made it into the ChibiOS 21.11.1 release, but any attempts to upgrade the code to that release are currently blocked at least on ChibiOS-Contrib not having support for 21.11.1). In general, QMK does not cherry pick patches into ChibiOS (but in this particular case the required patch seems to be already in the 20.3.x stable branch, just not in a release).

[exentio]

I had thought of making two different PRs but I wasn't sure about it, I can split them if you think it'd be better to do it.
I made a pull request to the ChibiOS repo (qmk/ChibiOS#1) since the changes are very small, but I don't know how often or for which reasons it gets updated.

[zvecr]

Also, qmk/ChibiOS#1 wont be accepted, as we do not want to maintain any cherry picks. Any changes will have to target upstream, and picked up when we bump chibios in a given develop cycle.

[stale]

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