stm32g0: Add flash bank swap support

[jsanchez-2g]

Summary

Add STM32G0 flash APIs required to switch dual-bank memory mapping and reload
flash option bytes:

• stm32_flash_swapbanks() toggles FLASH_OPTR_NSWAP_BANK
• stm32_flash_optload() launches an option-byte reload
• Bank swapping returns -EPERM when FLASH_SECR_BOOT_LOCK is active
• The swap operation is a no-op on single-bank devices

Reloading option bytes after changing the bank mapping normally causes an
immediate system reset.

Validation

• git diff --check: passed
• tools/checkpatch.sh -g HEAD^..HEAD: passed
• STM32G071 single-bank configuration with CONFIG_STM32_PROGMEM=y:
  • Flash driver compiled successfully
  • Both APIs exported
  • Single-bank no-op path compiled
• STM32G0B1 dual-bank configuration with CONFIG_STM32_PROGMEM=y:
  • Flash driver compiled successfully
  • Both APIs exported
  • Dual-bank swap path compiled

Hardware validation

Tested the equivalent existing implementation on a 2G BMS Golgi STM32G0B1
target using two consecutive RS-485 firmware updates.

Observed option-byte states:

1. Initial USB DFU image:
   • FLASH_OPTR=fef9feaa
   • nSWAP_BANK=1
2. After the first firmware update:
   • FLASH_OPTR=fee9feaa
   • nSWAP_BANK=0
3. After the second update, initiated while the banks were swapped:
   • FLASH_OPTR=fef9feaa
   • nSWAP_BANK=1

Both updates completed successfully. After each update, the target rebooted,
mounted its filesystem, initialized its communication interfaces and BMS
hardware, and resumed normal operation.

The hardware image used the product-tree implementation from which these APIs
were ported. The submitted code was rebased and compile-validated against
current Apache NuttX master.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb