Add STM32L151xC support

[htv04]

Description

This PR adds STM32L151xC support as groundwork for #26329, a Razer BlackWidow 2019 port (STM32L151RCT6). The platform code has been exercised on that hardware, and the included handwired/onekey/stm32l151 test board compiles with both default and reset keymaps.

The GENERIC_STM32_L151XC board is based on ChibiOS's ST_NUCLEO64_L152RE board and its demo mcuconf.h. Nucleo hardware assumptions were removed (e.g. HSE bypass, user LED/button, Arduino pin names), memory resized in the linker script, HSE/PLL clocking and USB enabled.

Unassigned pins are parked analog/floating rather than the Nucleo's defaults, except for PA11/PA12 (USB DM/DP). These must stay input-floating as the L1's USB pins aren't AF-muxed, and analog mode disconnects them from the USB peripheral.

eeprom_stm32_L0_L1.h included eeconfig.h for EECONFIG_SIZE, which has since moved to nvm_eeprom_eeconfig_internal.h. This causes L0/L1 builds to fail, so the header was updated as necessary.

Types of Changes

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