[Bug] Large external EEPROM fails on AVR

Describe the Bug

All the eeprom function signatures provided by avr-libc accept a void* address parameter for the EEPROM offset. This causes problems with AVR when attempting to address 32kB (using signed offsets) or 64kB (when using unsigned).

ChibiOS/ARM reuse the same function signatures to ensure compatibility across the rest of QMK -- these have no issue with addressing as the pointer size is 32-bit.

Suggested fix

• Rework the persistent storage as a separate subsystem, perhaps called nvram_read_byte() and similar, which provides the same set of functions, but with uint32_t offsets.
• Relocate things like the Teensy SmartEEPROM to match the current drivers/eeprom/ structure.
• Relocate the emulated flash to the same location.