Add support for analog USBPD on STM32G4xx.

[tzarc]

Description

Adds support for analog mode USBPD on STM32G4xx.
This does not include the full state machine implementation required to allow for anything higher than 5V/3A.
5.1k resistors are no longer required on designs, as long as the dead battery signals are connected to the CC pins on the MCU, and connected to the CC pins on the USB connector.

For board designers: it is highly suggested that ESD protection is also applied to the CC lines as well as the D+/D- lines.

For @qmk/collaborators: I've intentionally left out documentation as this is intimately tied to chip selection and board design. People unknowingly playing with this subsystem can eventually cause damage to PCB traces.

Supporting config screenshots:

https://www.st.com/content/ccc/resource/technical/document/technical_article/group0/59/a8/53/96/fe/a8/45/35/DM00496853/files/DM00496853.pdf/jcr:content/translations/en.DM00496853.pdf

https://www.st.com/resource/en/reference_manual/dm00355726-stm32g4-series-advanced-armbased-32bit-mcus-stmicroelectronics.pdf

Note Ra is effectively the in-cable 5.1k pull-down, due to cheaper cables usually only including one CC line. On the sink side, Ra is connected to GND through the internal Rd when using the USBPD peripheral, so a reading of vRa=0 should also signify base USB current capability. Some (most?) cheap cables seem to connect both CC lines to the same conductor in the cable, so the max is taken between both readings.

Types of Changes

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

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).