wifi: mt76: add support for providing eeprom in nvmem cells

Add support for providing eeprom in nvmem cells by adding nvmem cell as
an alternative source for mt76_get_of_eeprom().

Nvmem cells will follow standard nvmem cell definition and needs to be
called 'eeprom' to be correctly identified.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>